response will be undefined for 404 endpoint

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

Read lib/Requestable.js at line 187, then reproduce the issue by invoking getSha() for a nonexistent endpoint in the repository. Confirm the 404 path no longer leaves response undefined and verify the relevant behavior with the project's existing tests.

索引モデルが issue の本文から書いたものです。