mergePullRequest(number, options, cb) {
      return this._request('PUT', `/repos/${this.__fullname}/pulls/${number}/merge`, options, cb);
   }
Merge a pull request
@see https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button
@param {number|string} number - the number of the pull request to merge
@param {Object} options - the merge options for the pull request
@param {Requestable.callback} [cb] - will receive the merge information if the operation is successful
@return {Promise} - the promise for the http request