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/

--- a/CHANGES.txt	Sat Jun 01 03:52:02 2024 -0400
+++ b/CHANGES.txt	Sun Jun 02 18:22:53 2024 -0400
@@ -161,6 +161,9 @@
   rather than reporting a TypeError. (John Rouillard)
 - Make Last-Modified header use GMT not -0000 timezone. Fix error
   reported by redbot testing. (John Rouillard)
+- Send Vary: Accept-Encoding on any file that could be compressed
+  even if the file is not encoded/compressed. Found by Redbot
+  testing. (John Rouillard)
 
 Features: