Ensure shipped shipments are "immutable"

At least, in terms of the state. We shouldn't mark shipped shipments as
cancelled even if all of the items in the shipment get cancelled.

If a shipment has been marked as shipped, then we need to preserve that
state.

Author: adammathys

core/app/models/spree/shipment.rb

@@ -221,10 +221,10 @@ def determine_state(order)
     # ready      all other cases
     def recalculate_state
       self.state =
-        if order.canceled? || inventory_units.all?(&:canceled?)
-          "canceled"
-        elsif shipped?
+        if shipped?
           "shipped"
+        elsif order.canceled? || inventory_units.all?(&:canceled?)
+          "canceled"
         elsif !order.can_ship?
           "pending"
         elsif can_transition_from_pending_to_ready?