code: function _merge (getKeys, a, b) {
      return reduce([a, b], function (result, source) {
          forEach(getKeys(source), function (key) {
              result[key] = source[key];
          });
  
          return result;
      }, {});
  }
docs: Merges the received objects using the provided function to retrieve their keys.
  @private
  @param {Function} getKeys
  @param {Object} a
  @param {Object} b
  @returns {Function}