fix: Send Vary: Accept-Encoding on any data that could be compressed This allows upstream caches to return the correct data even when compression is not in use. It is not sent if the content would never be compressed. I.E. size < 100 bytes, dynamic compression disabled, file would not benefit from compression (img/jpeg, img/png). Fix setVary to add header to vary list only if it's not already there. Found by redbot.org testing. References: https://www.stackpath.com/blog/accept-encoding-vary-important/