Merge quantity of units when serializing shipment.items

Currently when receiving that shipment back via a webhook from wombat
the extention would complain about a diff in the items.

e.g. The received shipment items do not match with the shipment, diff: [{:sku=>"123", :quantity=>1}, {:sku=>"123", :quantity=>1}]

Before:

        items: [{ product_id: 1, quantity: 1 }, { product_id: 1, quantity: 1 }]

Now:

        items: [{ product_id: 1, quantity: 2 }]

// @JDutil @peterberkenbosch could you please review?

I think this is ok given the shipment.items key would be a representation of what we want in wombat and not what we have in spree exactly (inventory units don't have a quantity in spree).
yea the new exchanges. they have an inventory unit for the new variant, but don't add a new line item because that would add price to the order. they associate to the old line item so that if the new inventory unit gets returned we know how much money to give back