code: public function withAddedHeader($name, $value)
      {
          $response = clone $this;
  
          if ($this->isAllowedHeader($name = strtolower($name))) {
              $value = $this->getHeaderValue($value);
              $header = $this->getHeader($name);
              $response->headers[$name] = array_merge($header, $value);
          }
  
          return $response;
      }
docs: {@inheritdoc}