getKey(id, cb) {
      return this._request('GET', `/repos/${this.__fullname}/keys/${id}`, null, cb);
   }
Get a deploy key for the repository
@see https://developer.github.com/v3/repos/keys/#get-a-deploy-key
@param {number} id - the id of the deploy key
@param {Requestable.callback} cb - will receive the details of the deploy key
@return {Promise} - the promise for the http request