Implement WAC-Allow header

[srosset81]

Currently we find about the rights we have on a resource by fetching the whole ACL resource (and we don't use the Link header, which make the code incompatible with Solid).

But there exists a header to know about the rights on the resource !

http://solid.github.io/web-access-control-spec/#wac-allow

This means we would only need to GET the resource, and we could know faster if we have write permission, for example.

The data provider should still fetch the ACL resource if no WAC-Allow header is provided.