unbind before updateAll (typeo)
CherryPicked @jamiermill change for multi-group

Fixed a bug where updating a record and changing a group field to NULL would cause the behavior to ignore the new group value for ordering purposes.

This was because isset($model->data[$model->alias][$groupField) returns false if the value is NULL, even though the key *is set*. Fixed by using array_key_exists() instead.
Conflicts:
	tests/cases/behaviors/sequence.test.php