Fix callback in LineItem

#changed? here was not working as before, and the code was never getting past this guard in various places where previously it did. This meant the callback was effectively disabled, and order.update! was not being called when it should be.

app/models/spree/line_item.rb

@@ -226,7 +226,7 @@ module Spree
     end
 
     def update_order
-      return unless changed? || destroyed?
+      return unless saved_change_to_quantity? || destroyed?
 
       # update the order totals, etc.
       order.create_tax_charge!