Update file: handle case if no file exists at the path, or the response is something other than a file

[gr2m]

See https://developer.github.com/v3/repos/contents/#get-repository-content

The GET /repos/:owner/:repo/contents/:path endpoint can have 4 different types of responses

1. A file
2. A directory
3. A Symlink
4. A Submodule

It can also respond with a 404 nothing exists at the given path.

We should probably throw errors for response types 2-4, and pass null to the function in case of a 404 response

See CONTRIBUTING.md for instructions on how to record fixtures for a new test, if needed