`Spree::OrderContents#update_or_create` is used to update the cart when
on the /shop page. If you start an order and proceed to the "Order
summary" step, and then decide to update your order by using the shop
link next to the cart, such update wouldn't update the shipment.
This result in the order page in the backoffice displaying the wrong data,
and more importantly, in the stock not being updated.
So now we ensure shipment will be updated, which result in the checkout
flow being restarted, thus making sure the shipment is updated.
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