UpdateShipment has incomplete error message if shipment items do not match

The code [here](https://github.com/spree/spree_wombat/blob/master/lib/spree/wombat/handler/update_shipment_handler.rb#L83-L89), verifies that received shipment items is a superset of the shipment in the order, but it does not check that the sets are equal.

So if I order product A and B and only product A gets shipped, then Spree will be told that the entire order was shipped when it was not.
