code: protected function emit(ServerRequestInterface $request)
      {
          $response = $this->handle($request);
  
          $code = (string) $response->getStatusCode();
  
          $code .= ' ' . $response->getReasonPhrase();
  
          $headers = (array) $response->getHeaders();
  
          $version = $response->getProtocolVersion();
  
          header('HTTP/' . $version . ' ' . $code);
  
          foreach ($headers as $name => $values) {
              $value = (string) implode(',', $values);
  
              header((string) $name . ': ' . $value);
          }
  
          return $response;
      }
docs: Emits the headers based from the response.
  NOTE: To be removed in v1.0.0. Should be included in run().
  
  @param  \Psr\Http\Message\ServerRequestInterface $request
  @return \Psr\Http\Message\ResponseInterface