BAGGAGE HAVING A SELF-LOCKING HINGE The present invention relates to baggage. Baggage is known that has a base or lower shell serving to receive belongings. The base is closed by a lid. The lid is 'hinged to the base about a pivot axis, generally by means of two hinges. In particular, Document GB 2 323 591 discloses such a piece of baggage or container in which each of the hinges includes spring means exerting a force between the base and the lid, urging the lid to pivot from a closed position to an open position. The lid pivoting on the hinges is controlled by friction between two surfaces, one of the surfaces being constituted by a cap secured to or integral with the base, while the other surface is part of or is connected to the lid. The friction tends to oppose opening of the lid. The friction means comprises friction surfaces pressed together. The friction force between the friction surfaces is adjusted with a screw. But when the surfaces wear out, the friction ■force changes and needed to be adjusted again with the screw. A purpose of the invention is to mitigate, at least partially, such a drawback. To this aim, the invention provides a baggage comprising a base closed by a lid hinged to the base about a pivot axis by means of at least one hinge. This hinge includes friction means exerting a friction force tending to prevent, for either pivoting sense, the lid from pivoting relative to the base. It also includes spring means. Further, in this baggage, the friction means comprises two friction surfaces and the spring means exerts a resilient force pressing the friction surfaces together. Consequently, the spring means presses the friction surfaces together whatsoever the wear of the friction surfaces may be. In certain embodiments of the invention, it is possible optionally to use one or more of the following provisions : - the spring means exerts a resilient force urging the lid to pivot relative to the base. - the friction surfaces are cam surfaces: at least one of which tends to move along the pivot axis as the lid rotates about the pivot axis; - the hinge includes at least a moving ring and a fixed ring; the moving ring is adapted to move along the pivot axis; each of the fixed and moving rings is provided with a cam surface that is substantially helical about the pivot axis; the resilient force exerted by the spring means then pushes the moving ring against the fixed ring; this makes it possible to transform, via the cam surfaces, the movement in translation of the moving ring into a relative movement in rotation between the moving ring and the fixed ring; this makes it possible to obtain an increased friction force between the moving ring and the fixed ring, as the lid is opened; the spring means exerts a resilient force tending to open the lid; then, the lid can be opened easily by means of the resilient force that urges it away from the base; by means of the friction between the moving and the fixed rings, the lid is also positioned stably in any position, between its closed position and its open position; in other words, a user can have the lid held in a self-locked position regardless of the angle at which the lid is opened; thus, the lid cannot re-close in untimely manner; - the baggage includes two moving rings mounted to move along the pivot axis; the moving rings tend to be urged apart by the spring means; - the hinge is made up of a first hinge element and of a second hinge element; the first hinge element being secured to the base and being formed integrally with each fixed ring, and the second hinge element being secured to the lid and including a channel having at least one opening in which a slide is engaged that is secured to the moving ring, the slide being prevented from moving in rotation in the channel; - the first hinge element and the second hinge element come into abutment when the lid is closed on the base; - the lid is locked in an open position in which it extends substantially at 90° relative to the base, by means of the moving ring and of the fixed ring; - the spring means . comprise a spiral spring or a pneumatic actuator; and - the moving rin,g and the fixed ring are made of polyamide 6-6 filled with 50% by weight of glass fibers; it is thus possible to obtain a good compromise between a large coefficient of friction and a small amount of wear; such a material is marketed by several manufacturers and can be easily found by a skilled person. Other aspects, objects, and advantages of the invention will appear on reading the following description of one of these embodiments. The invention will also be better understood with reference to the drawings, in which: Figure 1 is a diagrammatic perspective view showing an embodiment of baggage of the present invention, with a single hinge; Figure 2 is a diagrammatic perspective view showing an embodiment of a hinge, in the open position, for the baggage shown in Figure 1; and Figure 3 is a diagrammatic perspective view in the opposite direction showing a detail of the hinge shown in Figure 2. In the various figures, like references designate elements that are identical or similar. In an embodiment of the baggage 1 of the present invention, shown in Figure 1, the baggage comprises a base 2 and a lid 4. The base 2 and the lid 4 are both made of a rigid material. The base 2 has a bottom wall 6 and four peripheral walls 8, each of which extends respectively substantially perpendicularly to the bottom wall 6. Thus, the base 2 forms a receptacle adapted for receiving belongings. Each of two of the peripheral walls 8 is provided with a respective small wheel 10 making the baggage 1 easier to move. The lid 4 is hinged to another peripheral wall 8 situated, in this example, opposite from the handle 100, by means of a single hinge 12. As shown in Figure 2, the hinge 12 is made up of first and second hinge elements 14, 16. The first hinge element 13 is designed to be connected to the lid 4, while the second hinge element 16 is designed to be fixed to the base 2. The first and second hinge elements 14, 16 are screwed or bonded by adhesive respectively to the lid 4 and to the base 2. The first and second hinge elements 14, 16 are mounted to pivot relative to each other about a pivot axis X. A metal and rigid rod 18 extends along the pivot axis X. The first and second hinge elements 14, 16 are held together by the rod 18. The first hinge element 14 has two plates 20, 22 connected together via a curved swan-neck-shaped panel 24, i.e. substantially shaped as a cylinder portion. The plate 20 is designed to be fixed to the lid. It extends radially outwards with reference to the curvature of the panel 24. The plate 22 is designed to be articulated to the second hinge element 16. It extends radially inwards with reference to the curvature of the panel 24. The pivot axis X is substantially comprised in the plane of the plate 22. The panel 24 and the plate 20 are visible at least in part when the lid 4 is open. Therefore, they can be covered with a trim 26. The plate 22 is extended at either longitudinal end ("longitudinal" referring to the direction parallel to the pivot axis X) by two respective tabs 28, themselves extending perpendicular to the pivot axis X, and each of which has its end provided with a respective external ring 30. The external rings 30 are formed integrally with the plates 20, 22, the panel 24, and the tabs 28. Then, each external ring 30 is fixed with regard to the ' first hinge element 14. The rod 18 passes through each external ring 30 and extends substantially parallel to the pivot axis X. Thus, the first hinge element 14 is hinged to the rod 18 by the external rings 30. As shown in Figu're 3, each external ring 30 has a cam surface 32 that is substantially helical around the pivot axis X. In this embodiment, the rotation direction of the helical cam surface 32 is chosen in order to make easier the lid opening, but as a variant the other direction would be chosen. Each 'external ring 30 is also provided with a setback 34 extending substantially over 90° about the pivot axis X, between two walls 36 extending radially and perpendicularly to the pivot axis X. Returning to Figure 2, the second hinge element 16 has a plate 23 having a respective tab 38 at each of its longitudinal ends ("longitudinal" referring again to the direction parallel to the pivot axis X) . The rod 18 passes through each tab 38. Thus, the rod 18 is supported by the plate 23, which is itself fixed to the base 2. In a central zone, the plate 23 has a channel 40. The pivot axis X passes longitudinally through the channel 40. An external ring 30 connected to the first hinge element 14 and an internal ring 42 are interposed between the channel 40 and each tab 38. Each internal ring 42 is formed integrally with a slide 48 and with a tab 50. As shown in Figure 3, each internal ring 42 has a cam surface 44 that is substantially helical about the pivot axis X and complementary to the cam surface 32. Said cam surface 44 bears against the above-described cam surface 32 while the first hinge element 14 pivo.ts relative to the second hinge element 16. In Figures 2 and 3, the cam surfaces 32 and 44 are shown spaced apart from each other for reasons of clarity. The moving ring 42 has an abutment 46 with a surface extending radially ■ relative to the pivot axis X and longitudinally parallel thereto. While the first hinge element 14 is pivoting relative to the second hinge element 16, when the baggage is opened, a portion of the moving ring 42 penetrates into the setback 34. When the lid 4 is in a complete open position, the abutment 46 bears against one of the walls 36 of each above-described external ring 30. Thus, the first hinge element 14 is prevented from pivoting in the opening direction relative to the second hinge element 16. When the first and second hinge elements 14 and 16 are in this relative position, the lid 4 extends substantially at 90° relative to the base 2, e.g. at 94°. Each internal ring 42 is extended parallel to the pivot axis X by a respective slide 48. The slide 48 has a cross-section, perpendicular to the pivot axis X, that is substantially square. Thus each slide 48 fits into the complementary shape of the channel 40, in which it is prevented from moving in rotation around the pivot axis X. Each internal ring 42 can thus only move along the pivot axis X, in a way while the lid 4 is opening and in the opposite way while the lid 4 is closing. Returning to Figure 2, each internal ring 42 has a tab 50 extending radially towards outside, relative to the pivot axis X. Spring means 52, such as a spiral spring, for example, are interposed between the two tabs 50. While the first hinge element 14 is pivoting relative to the second hinge element 16, while the lid 4 is opening or closing, the two internal rings 42 move towards or away from each other. The spring means ■ 52 exert a force between the two tabs 50 tending to move them apart. In the example hereby described, while 'the lid is closing, the two internal rings 42 move towards each other while exerting, a force opposite from the force delivered by the spring means 52. When the lid 4 is in the closed position, the plate 22 is in abutment against the second hinge element 16. The second hinge element 16 has a cover 54 masking the spring means 52 when the hinge 12 is mounted on the baggage 1 (see Figure 1) . The friction forces between the cam surfaces 32 and 44 oppose the resilie'nt force exerted by the spring means 52 tending, via said cam surfaces 32, 44, to open the lid 4. According to a preferred embodiment of the invention, thanks to a sensible choice of, and/or thanks to an adjustment, of the resilie'nt force of the spring means 52, taking into account the material nature constituting the cam surfaces 32, 44 and of the helical profile of these surfaces, a balance can be obtained between, on the one hand, the friction force between the cam surfaces 32, 44 and the lid weight and, on the other hand, the resilient force exerted by the spring means 52. Such a balance is obtained whatever the lid position between the closing and opening positions. To this aim, the spring means are pre-stressed to various extents. It is possible to adjust the prestress as a function of the models, so as to keep the same spring means 52. For example, the prestress is adjusted by interposing spacers between the spring means 52 and the tabs 50. The first hinge element 14 is made of polyamide 6-6 filled with 50% by weight of glass fibers. The rings 42 are made of polyamide 6-6 filled with 50% by weight of glass fibers. This material is adapted to obtain the desired friction force between the cam surfaces of the rings 30 and 42 while also withstanding wear. In the example hereby described, for a- steepness of the plane inclination of the cam surfaces relatively to the pivot axis X substantially equal to 45 degree, the elastic stiffness is chosen substantially equal to 11.8 newtons per millimeter (N/mm) , for the resilient force exerted by the spring means 52. In addition, even if stowage means are provided in the lid 4, the hinge 12 retains its self-locking properties even with a load of 1.5 kilograms (kg) placed in the stowage means (the total weight of the lid and of the load can then reach 3 kg, for example) . Numerous variants can be imagined in the manner of the above-described embodiment of the baggage. Thus, for example, a wedging washer made of Teflon" can be provided between one of the external rings 30 and the adjacent tab 38. Similarly, the spring means 52 can be constituted by a pneumatic jack. In other variants, two or more hinges 12 equip the baggage 1.