Update fos_http_cache.yml

Author: andrerom

src/Resources/config/fos_http_cache.yml

@@ -12,3 +12,4 @@ tags:
     # Disbled on FOSHttpCache 1.x as it does not support usage on anything but varnsih.
     # @todo Enable and configure for 2.x where we are able to configure tag formatter
     enabled: false
+    header: xkey