Fix Gist spec for vnd.github.v3 media type response.

Changed gist.user to gist.owner references for vnd.github.v3 media type.
Add call to star_gist in before method for .gist_starred? test.
Merge pull request #421 from joeyw/fix-gist-spec

Fix Gist spec for vnd.github.v3 media type response.