Fix RSS feeds not being cachable (#14368)

* Add tests for some cachable responses

This only covers responses that we should have managed to make cachable
so far. It's not the case of all responses that should be cachable in
the end.

* Fix RSS feeds not being cachable

app/controllers/application_controller.rb

@@ -55,7 +55,7 @@ class ApplicationController < ActionController::Base
   end
 
   def store_current_location
-    store_location_for(:user, request.url) unless request.format == :json
+    store_location_for(:user, request.url) unless [:json, :rss].include?(request.format&.to_sym)
   end
 
   def require_admin!