code: protected function finalize($function)
      {
          $response = $this->container->get(self::RESPONSE);
  
          if (is_string($function) === true) {
              $stream = $response->getBody();
  
              $stream->write((string) $function);
          }
  
          $instanceof = $function instanceof ResponseInterface;
  
          return $instanceof ? $function : $response;
      }
docs: Converts the result into a \Psr\Http\Message\ResponseInterface instance.
  
  @param  \Psr\Http\Message\ResponseInterface|mixed $function
  @return \Psr\Http\Message\ResponseInterface