Update guard clause in line item callbacks (take two)

The `changed?` method is more accurately replaced by `saved_changes.present?`

In some cases here (where the object had just been saved for the first time) the guard clause was still stopping execution unexpectedly.

app/models/spree/line_item.rb

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