response will be undefined for 404 endpoint

[sajithdilshan]

https://github.com/github-tools/github/blob/24223f7a51ba935f1c86ffd552dac20e8f3e359d/lib/Requestable.js#L187

In the above code response will be undefined if the request is for a non-existing endpoint (which will result in 404 response).

In-order to re-produce invoke getSha() method for a non-existing in the repository