protected function getCacheControlHeader()
    {
        $cacheControl = '';
        foreach ($this->cacheDirectives as $cacheDirective) {
            $cacheControl .= ($cacheDirective !== '' ? $cacheDirective . ', ' : '');
        }
        $cacheControl = trim($cacheControl, ' ,');
        return ($cacheControl === '' ? null : $cacheControl);
    }