docs: function callHandler(fn, req, res, next) {
      let context = new Context(req, res);
      if (utils.isGeneratorFunction(fn)) {
          co(fn).call(context, function (err) {
              if (err || !res.terminated) {
                  next(err);
              }
          });
      } else {
          fn.call(context);
          if (!res.terminated) {
              next();
          }
      }
  }