function _stringifyJsonxError(err) {
    var plainObject = {};
    Object.getOwnPropertyNames(err).forEach(function (key) {
      plainObject[key] = err[key];
    });
    return JSON.stringify(plainObject);
  }
[_mixinJsonxMethod description]
@param  {[type]} req [description]
@param  {[type]} res [description]
@return {[type]}     [description]