Fix api checkouts_controller specs

Order state was being set to address before expected by the spec because
the order already had shipment. And that's the behaviour now. Order with
shipments should have their shipments removed and state set back do
address so that users go through delivery step again and shipments are
regenerated

Update line_items properly in Api::OrderControllers#update

When trying to update a line_item quantity, for example, using this
controller it would try to create a new line item instead. The fix was
already applied to master branch in 379a12275e21167c11582b1abf8052b7453973ad

Fixes #3621