`GoogleOAuth2Mixin.authorize_redirect` should differ from `OAuth2Mixin.authorize_redirect` only in google-specific ways.  We don't construct absolute urls in the base class or in the facebook mixin, so we shouldn't here either (I think this may mean that some doc examples are wrong).  `overwrites` should probably be named `extra_params` for consistency.  Are scope and response_type standard enough to include in the base class?  If so then I'd rather just get rid of this method so callers won't have to think about the inheritance relationship and what's different between the two.