Do not skip many2many in _changed_values

Like the one2many they should always be processed as they can have been
updated.

Closes #12474
(grafted from f45d89eb8048c28887d2e282a5dc7604bc8d9846)

--HG--
branch : 6.0

trytond/trytond/model/modelview.py

@@ -854,7 +854,7 @@ class ModelView(Model):
             # Always test key presence in case value is None
             if (fname in init_values
                     and value == init_values[fname]
-                    and field._type != 'one2many'):
+                    and field._type not in {'one2many', 'many2many'}):
                 continue
             if field._type in ('many2one', 'one2one', 'reference'):
                 if value: