Fix shipment persistence on order importer

Besides selecting the shipment rate we also need to assign the cost in
the shipment itself.

We cant only rely on order.update_shipment_state to actually well update
the order shipment_state (:O) because shipment states might not be sync
with the latest payment changes which influence on the result so
Shipment#determine_state needs to run to make sure we keep those
accurate

Conflicts:
	core/spec/lib/spree/core/importer/order_spec.rb