Added enhanceRequest option in grant parsing functions

Basically I needed a way to parse extra parameters in the grant request and store them in the `oauth2.req` object. The other way I found to do that was to override the parsing method, but it's ugly.

Moreover, I needed these parameters in the `next` function of `validate` in the `authorization` module, for error handling. So I moved the `req.oauth2.req = areq` just before the next.