intridea/grape — pull request #708 by etehtsea (2014-08-05 15:52:42+00:00)
Title: Fix oauth2 compatibility with draft-ietf-oauth-v2-31

http://tools.ietf.org/id/draft-ietf-oauth-v2-31.html#rfc.section.5.2
As far as I see status codes for `invalid_grant` should be `400`.