Improve assertion in convert_domain of Function field

The field name from the domain could be a nested if the Function field is
relational. So it must just if it starts with the name of the field.

trytond/model/fields/function.py

@@ -63,7 +63,7 @@ class Function(Field):
 
     def convert_domain(self, domain, tables, Model):
         name, operator, value = domain[:3]
-        assert name == self.name
+        assert name.startswith(self.name)
         method = getattr(Model, 'domain_%s' % name, None)
         if method:
             return method(domain, tables)