According to the XMLHttpRequest whatwg.org specification:
If the request method is GET or HEAD, set body to null.

When use _request method, data was not send to the GitHup API:
https://github.com/michael/github/blob/master/github.js#L525
https://github.com/michael/github/blob/master/github.js#L599

It works well on master branch 'cause GitHub API set ref to master by default.