Rebuild the Cache-Control HTTP header using the user-specified values
private function rebuildCacheControlDirective() {
$cacheControl = array();
foreach ($this->cacheControl as $key => $value) {
$cacheControl[] = $value === true ? $key : $key . '=' . $value;
}
$this->headers['cache-control'] = new Header('Cache-Control', $cacheControl, ', ');
}