code: function(refreshToken, callback) {
      var params = {
        grant_type : "refresh_token",
        refresh_token : refreshToken,
        client_id : this.clientId
      };
      if (this.clientSecret) {
        params.client_secret = this.clientSecret;
      }
      return this._postParams(params, callback);
    }
docs: @typedef TokenResponse
  @type {Object}
  @property {String} access_token
  @property {String} refresh_token
   
  OAuth2 Refresh Token Flow
  
  @param {String} refreshToken - Refresh token
  @param {Callback.<TokenResponse>} [callback] - Callback function
  @returns {Promise.<TokenResponse>}