This ensures we can still use Order#shipment although Spree deprecates
it, while fixing a bug at the same time. The problem that was making the
test fail was on `Order#shipment` that Spree defines.
If the shipments association changes, `#shipment` returns stale data.
That is because the order object we might be using is still alive, and
so its @shipment ivar still holds an old shipment object (it's not nil)
and thus `@shipment ||= shipments.last` doesn't evaluate the right-hand
side of the expression.
Note that we need to `prepend` the evaluation of the concern (which it's
been rename) for our methods to take precedence over Spree ones. With
`include`, Spree's `#shipment` would still be picked up making the test
fail.
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