protected function writer_headers()
 {
     if ($this->headers_written) {
         return;
     }
     $this->headers_written = true;
     $map = $this->map;
     if (!is_array($map)) {
         return;
     }
     $headers = array();
     foreach ($map as $key => $value) {
         $headers[] = $value;
     }
     parent::put($headers);
 }