protected function generateCacheHeader()
    {
        $cacheControl = [];
        foreach ($this->getDirectives() as $directive => $value) {
            if ($value === true) {
                $cacheControl[] = $directive;
            } else {
                $cacheControl[] = $directive . '=' . $value;
            }
        }
        return implode(', ', $cacheControl);
    }