Eliminate a ruby warning in a test in Action View

This eliminates the warning below:

  actionview/test/template/form_helper_test.rb:913: warning: method redefined; discarding old id_came_from_user?
  actionview/test/template/form_helper_test.rb:104: warning: previous definition of id_came_from_user? was here

actionview/test/template/form_helper_test.rb

@@ -910,7 +910,11 @@ def test_inputs_use_before_type_cast_to_retain_information_from_validations_like
   end
 
   def test_inputs_dont_use_before_type_cast_when_value_did_not_come_from_user
-    def @post.id_came_from_user?; false; end
+    class << @post
+      undef id_came_from_user?
+      def id_came_from_user?; false; end
+    end
+
     assert_dom_equal(
       %{<textarea id="post_id" name="post[id]">\n0</textarea>},
       text_area("post", "id")