Fix caching headers in ActivityPub endpoints (#11331)

* Fix reverse-proxy caching in public fetch mode

* Fix caching in ActivityPub-specific controllers

app/controllers/application_controller.rb

@@ -160,6 +160,6 @@ class ApplicationController < ActionController::Base
   end
 
   def set_cache_headers
-    response.headers['Vary'] = 'Accept, Signature'
+    response.headers['Vary'] = public_fetch_mode? ? 'Accept' : 'Accept, Signature'
   end
 end