code: public function process(ServerRequestInterface $request, HandlerInterface $handler)
      {
          $middleware = $this->middleware;
  
          if ($this->response !== null)
          {
              $handler = function ($request) use ($handler)
              {
                  return $handler->{HANDLER_METHOD}($request);
              };
  
              $response = $this->response;
  
              return $middleware($request, $response, $handler);
          }
  
          return $middleware($request, $handler);
      }
docs: Processes an incoming server request and return a response.
  
  @param  \Psr\Http\Message\ServerRequestInterface      $request
  @param  \Rougin\Slytherin\Middleware\HandlerInterface $handler
  @return \Psr\Http\Message\ResponseInterface