writeFile function fails when response status header is 304 unmodified

[RobiFerentz]

The response object in the getSha returned promise fails on here commit.sha = response.data.sha; The response.data is null, the status header of the response object is '304 Unmodified' The writeFile function fails before the PUT requests are created.

https://github.com/github-tools/github/blob/24223f7a51ba935f1c86ffd552dac20e8f3e359d/lib/Repository.js#L732