Avoid overwriting notifications for template assertions.

test/fixtures/template.html.erb

@@ -0,0 +1 @@
+<p>Hello.</p>

test/integration/action_controller/serialization_test_case_test.rb

@@ -11,6 +11,11 @@ module ActionController
         def render_text
           render text: 'ok'
         end
+
+        def render_template
+          prepend_view_path "./test/fixtures"
+          render template: "template"
+        end
       end
 
       tests MyController
@@ -56,6 +61,11 @@ module ActionController
         end
         assert_match 'assert_serializer only accepts a String, Symbol, Regexp, ActiveModel::Serializer, or nil', e.message
       end
+
+      def test_does_not_overwrite_notification_subscriptions
+        get :render_template
+        assert_template "template"
+      end
     end
   end
 end