public function push($handler)
    {
        if (!$handler) {
            return $this;
        }
        if ($this->next) {
            return $this->next->push($handler);
        }
        if (!$handler instanceof MiddlewareInterface) {
            $handler = new Middleware($handler);
        }
        $this->next = $handler;
        return $this->next;
    }