This pull request removed the override of `process_payments!` which was based on v1. Spree v2 has an additional check: An order in payment state requires a payment. Some specs didn't care and didn't create payments before transitioning to `complete`.
- fixed order creation to use order.shipments instead of order.shipping_method
- adapted test to new spree 2 controller logic (shipments page is gone since 67f568914988bcc0a1fc520d15ed6444a6d12824 and redirect logic changed on e9cde1b4d570dd4f7f979ac71a58d6f3f342ebb4)