private function addPurgeAuthHeader(array $headers)
    {
        if ($this->configResolver->hasParameter(self::INVALIDATE_TOKEN_PARAM)
            && null !== ($token = $this->configResolver->getParameter(self::INVALIDATE_TOKEN_PARAM))
        ) {
            $headers[self::INVALIDATE_TOKEN_PARAM_NAME] = $token;
        }

        return $headers;
    }