[feature] GitHub CRUD tests

I filled in the test stubs for github_update_file, github_delete_file, and github_view_file.

This replaces [PR 1243](https://github.com/CenterForOpenScience/osf/pull/1243) in the old repo, which I am going to close now.

This closes [PR 1278](https://github.com/CenterForOpenScience/osf.io/pull/1278).
A useful assertion here might be to test that the current path is the GUID url for the file:

```python
        assert_equal(res2.request.path, '/{0}/'.format(guid._id))
```