/**
* Return a sorted list of Vary headers.
*
* While headers are case-insensitive, header values are not. We can only
* normalize the order of headers to combine cache entries.
*
* @param ResponseInterface $response The Response with Vary headers.
*
* @return array An array of sorted headers.
*/
private function normalizeVary(ResponseInterface $response)
{
$parts = AbstractMessage::normalizeHeader($response, 'vary');
sort($parts);
return $parts;
}
