code: function wrapMiddleware(fn) {
    assert(typeof fn === 'function', `express中间件必须是一个函数类型`);
    assert(fn.length >= 2, `express中间件必须至少有2个参数`);
    if (fn.length === 2) {
      return function (req, res, next) {
        let p = null;
        try {
          p = fn(req, res);
        } catch (err) {
          return next(err);
        }
        p.then(_ => next()).catch(err => next(err));
      };
    }
    return fn;
  }
docs: 封装中间件
  
  @param {Function} fn
  @return {Function}