protected function getCacheControlHeader() {
$parts = array();
ksort($this->cacheControl);
foreach ($this->cacheControl as $key => $value) {
if (true === $value) {
$parts[] = $key;
}
else {
if (preg_match('#[^a-zA-Z0-9._-]#', $value)) {
$value = '"' . $value . '"';
}
$parts[] = "{$key}={$value}";
}
}
return implode(', ', $parts);
}