Use response_vary_headers method in requests/cache_spec (mastodon#2…

…9411)

spec/requests/cache_spec.rb

@@ -184,7 +184,7 @@ module DisabledAnonymousAPI
         get '/users/alice'
 
         expect(response).to redirect_to('/@alice')
-        expect(response.headers['Vary']&.split(',')&.map { |x| x.strip.downcase }).to include('accept')
+        expect(response_vary_headers).to include('accept')
       end
     end