Always test field presence in initial value when computed changed values

The value could be None which is also the default value returned by dict.get.
So it will fail to detect change in such case.

issue4839
review23281002

trytond/model/modelview.py

@@ -689,7 +689,9 @@ class ModelView(Model):
             return changed
         for fname, value in self._values.iteritems():
             field = self._fields[fname]
-            if (value == init_values.get(fname)
+            # Always test key presence in case value is None
+            if (fname in init_values
+                    and value == init_values[fname]
                     and field._type != 'one2many'):
                 continue
             if field._type in ('many2one', 'one2one', 'reference'):