purge: Prevent caching of purge response

Especially since our stricter CDN ignores query parameters, and thus
makes it impossible to purge two different URLs since the second
hit will serve the response to the previous purge request.

Ref https://github.com/jquery/infrastructure/issues/474.

Author: Krinkle

purge.php

@@ -77,6 +77,7 @@ function jq_request_post_json( $url, array $headers, array $postData ) {
 }
 
 header( 'Content-Type: text/plain' );
+header( 'Cache-Control: private, no-cache, must-revalidate' );
 header( 'X-Content-Type-Options: nosniff' );
 echo "Attempting to purge:\n{$file}\n\n";