Add method for authenticated user path

Small and maybe insignificant refactor to `lib/octokit/client/users.rb` to remove some duplication.

Originally I wanted this method to reside in `lib/octokit/client.rb` for use in [blocks like this][1], which I think occurs in a few other places, however I found the authenticated Gists path to be different:

- `GET /user/starred` for repositories being starred by the authenticated user
- `GET /user/subscriptions` for repositories being watched by the authenticated user

vs. 

- `GET /gists` for authenticated user’s gists 

[1]: https://github.com/octokit/octokit.rb/blob/master/lib/octokit/client/gists.rb#L18-L24