[IMP] point_of_sale: 'Receipt Ref code' when returning product on order

It's about 'Receipt Reference code' which gets generated when Order is
created from POS .

After this, click on 'RETURN PRODUCTS' button, the return Order doesn't
fetch the "Receipt Ref" of the origin Order, so there is no way to find
the origin order from where the return of the products done.

addons/point_of_sale/models/pos_order.py

@@ -631,6 +631,7 @@ class PosOrder(models.Model):
                 'name': order.name + _(' REFUND'),
                 'session_id': current_session.id,
                 'date_order': fields.Datetime.now(),
+                'pos_reference': order.pos_reference,
             })
             PosOrder += clone