append .xmlschema to system hook timestamp

app/services/system_hooks_service.rb

@@ -18,7 +18,7 @@ class SystemHooksService
   def build_event_data(model, event)
     data = {
       event_name: build_event_name(model, event),
-      created_at: model.created_at
+      created_at: model.created_at.xmlschema
     }
     case model