private function makeHeaderValue()
    {
        $pieces = ['max-age=' . $this->config['max-age']];

        if ($this->config['subdomains'])
        {
            $pieces[] = 'includeSubDomains';
        }

        if ($this->config['preload'])
        {
            $pieces[] = 'preload';
        }

        return implode('; ', $pieces);
    }