Treat response with 201 status code as success (required for social network XING)

During the authentication flow, social business network [XING](http://www.xing.com) responds with a 201 ("Created") http status code instead of the usual 200 ("OK").

Currently go-oauth treats every status code other than 200 as an error. How about allowing 201 as well? A simple one liner seems to do the trick...

Thanks,
Ulf