Add support for private and public Cache-Control attributes

[Florimond Manca]

While #4 will add support for adding arbitrary Cache-Control attributes, the private and public attributes are a bit special. They are exclusive, in that only one of them should be present in the Cache-Control header. Besides, if private is present then the response should never be cached.

To implement this feature:

• Modify the Cache-Control header generation to allow passing private=True and public=True to the @cache_control header (resulting in the private or public directives being present in the Cache-Control header).
  • Make sure that only one of private or public is passed to @cache_control, or raise an exception (e.g. ValueError).
  • If one of public or private is already present in the Cache-Control header (this may happen if e.g. the view returned as response with Cache-Control: public), it should be overridden.
• In store_in_cache(), make sure to not store the response in the cache if the private cache-control directive is present.
• Add tests for the new behavior (and modify the one that was testing that private and public weren't supported yet).

Note that this can only be tackled once #4 has been resolved.

[Florimond Manca]

#4 is in, so we can start working on this issue. :)