The original payment may not be valid because its credit card may be
expired. Stripe gives this as a valid scenario returning a success and
we should do too.
When creating the credit payment we end up validating all sources in
a chain as follows.
```
Payment being persisted -> source payment -> original credit card.
```
The source payment was valid when created (It would not be persisted
otherwise) but its source card may now be expired, and that's legit.
There was also an issue with the `#invalidate_old_payments` callback. It
was causing the original payment to be validated again and thus the
credit payment failed to be persisted due to the original credit card
being expired. Switching this callback to use `#update_column` skips
validations and so we don't validate the source payment. We only care
about the state there, so it should be fine.
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)
