writeFile function fails when response status header is 304 unmodified

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

Start in lib/Repository.js at the linked writeFile code, tracing the getSha promise and the response handling around commit.sha = response.data.sha. Confirm the expected behavior for a 304 response and verify that writeFile no longer fails before creating its PUT requests.

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