Private data not returning

When making a call to get the authenticated user's gists, it only returns the public gists.

```
import GitHubAPI from 'github-api';

const getGists = (async () => {
var gh = new GitHubAPI({
token: 'token'
});
var user = gh.getUser();
var gists = user.listGists().then( data => data );

return gists;
})
```

Is this a GitHub API issue, a GitHub.JS issue, or am I missing something?

Start by reproducing the authenticated user.listGists() call from the issue with a token and inspect whether private gists are omitted. Compare the wrapper's response with the GitHub API behavior to identify whether the problem belongs to GitHub.JS or the API; done means the source of the omission and the expected behavior are established.

索引モデルが issue の本文から書いたものです。