OAuth2.0 fix

[bclozel]

Allow users to redefine OAuth2.0 token header key sent along resource requests. Default value is "Bearer".

Implement auto-renewal of the OAuth2.0 token when it's expired. I didn't implement this as a ResponseFilter because:

• RequestFilter and ResponseFilters are totally decoupled (so you can't modify the token store within the RequestFilter from the ResponseFilter)
• You can actuall receive HTTP Unauthorized responses for a lot of reasons (not only an expired token), and messing with the response can be an unpleasant surprise for RESThub's users
• A ResponseFilter can replay a request, but not issue a request AND replay the last request

Don't merge this pull request now. I'll probably modify it, I'm waiting for feedback on that matter.

[sdeleuze]

Thanks for these very useful improvements.

I am fine with the proposed improvement, just please add some tests for token renewal with Spring Secureity OAuth2 serverside. There are some existing OAuth2 integration tests that could be used for that by configuring a Spring Security OAuth 2 context with refresh token enabled.