Update model data also when old values are different from the current values

Closes #12190
(grafted from 4820f12259f64a26ada02af640231459fb6ab820)

--HG--
branch : 6.0

trytond/trytond/convert.py

@@ -762,7 +762,7 @@ class TrytondXmlHandler(sax.handler.ContentHandler):
             fs_values = old_values.copy()
             fs_values.update(new_values)
 
-            if values != fs_values:
+            if old_values != values or values != fs_values:
                 self.grouped_model_data.extend(([self.ModelData(mdata_id)], {
                             'fs_id': fs_id,
                             'model': model,