Replace use of assigns in test

`assigns` was removed from Rails and using `instance_variable_get` is
the new correct way to assign an instance variable in tests.

Author: eileencodes

test/caching_test.rb

@@ -563,7 +563,7 @@ def test_invalid_format_returns_not_acceptable
   private
 
     def content_to_cache
-      assigns(:cache_this)
+      @controller.instance_variable_get(:@cache_this)
     end
 
     def fragment_exist?(path)