Do not use defaultdict as changed value

xmlrpclib can not dump it and _changed_value is used by RPC on_change methods.

issue6943
review44431002

trytond/model/modelview.py

@@ -738,6 +738,7 @@ class ModelView(Model):
                     del value['remove']
                 if not value:
                     continue
+                value = dict(value)
             elif field._type == 'many2many':
                 value = [r.id for r in value]
             changed[fname] = value