OAuth2: support application/x-www-form-urlencoded replies
OAuth2: support application/x-www-form-urlencoded replies

The application/x-www-form-urlencoded content type for replies was used
in older versions of the standard, but there are still websites which
use it (like github.com).
Fixes: http://code.google.com/p/accounts-sso/issues/detail?id=134

plotters/accounts-sso.signon-plugin-oauth2,plotters/accounts-sso.signon-plugin-oauth2