Changes in oauth2:refresh_access_token

WARNING: BREAKS THE COMPATIBILITY WITH PREVIOUS CODE USING oauth2:refresh_acess_token AND PREVIOUS BACKEND IMPLEMENTATIONS.
- Adds Scope parameter to oauth2:refresh_access_token, see [point 6 of the specification](http://tools.ietf.org/html/rfc6749#section-6).
- Changes error codes returned by oauth2:refresh_access_token to match [point 5.2 of the specification](http://tools.ietf.org/html/rfc6749#section-5.2).