Better guess "selection_change_with" attribute

Testing on the instance for Selection field is not enough because some field
selection could come from a Function field.
So it is better to use a duck typing style in this case.

issue3649
review3451002

trytond/model/model.py

@@ -53,8 +53,8 @@ class Model(WarningErrorMixin, URLMixin, PoolBase):
             for attribute in ('on_change', 'on_change_with', 'autocomplete',
                     'selection_change_with'):
                 if attribute == 'selection_change_with':
-                    if (isinstance(field, fields.Selection)
-                            and isinstance(field.selection, basestring)):
+                    if isinstance(
+                            getattr(field, 'selection', None), basestring):
                         function_name = field.selection
                     else:
                         continue