Fix condition in _on_change_set method

If _on_change_set is called with a None value for a XXX2Many field, the test
designed to check for creations / updates raises an Exception because of
missing parenthesis.

issue4284
review10591002

proteus/__init__.py

@@ -921,7 +921,7 @@ class Model(object):
             for record in to_remove:
                 # remove without signal
                 list.remove(getattr(self, field), record)
-            if value and value.get('add') or value.get('update'):
+            if value and (value.get('add') or value.get('update')):
                 for index, vals in value.get('add', []):
                     relation = Model.get(self._fields[field]['relation'],
                             self._config)