public static function isRequestCacheable(Google_Http_Request $resp)
  {
    $method = $resp->getRequestMethod();
    if (! in_array($method, self::$CACHEABLE_HTTP_METHODS)) {
      return false;
    }

    // Don't cache authorized requests/responses.
    // [rfc2616-14.8] When a shared cache receives a request containing an
    // Authorization field, it MUST NOT return the corresponding response
    // as a reply to any other request...
    if ($resp->getRequestHeader("authorization")) {
      return false;
    }

    return true;
  }