Fix Adjustment#order= issue for non-order items

If you attempt to assign a non-order to this association, then it gives you a cryptic 'uninitialized constant Order' error. No fun at all! By specifying the class name, it'll show you the right error.

Related to #3856
Fixed issue where RenameAdjustmentFields was trying to assign non-orders to the adjustment.order association

Fixes #3856