updatePullRequest(number, options, cb) {
      return this._request('PATCH', `/repos/${this.__fullname}/pulls/${number}`, options, cb);
   }
Update a pull request
@see https://developer.github.com/v3/pulls/#update-a-pull-request
@param {number|string} number - the number of the pull request to update
@param {Object} options - the pull request description
@param {Requestable.callback} [cb] - will receive the pull request information
@return {Promise} - the promise for the http request