code: public function header($key, $value)
      {
          if (is_array($key) && !empty($key)) {
              foreach ($key as $k => $v) {
                  $this->headers->set($k, $v);
              }
          } elseif (is_string($key) && !empty($key)) {
              $this->headers->set($key, $value);
          }
          return $this;
      }
docs: Add custom header data in Response
  
  @param array|string $key
  @param string $value
  
  @return Response