Existing item stock levels not updated when adding or deleting from order on edit page

Scenario:

You are using uc_stock to manage stock levels for your store.

1. In the order edit screen, change the quantity for an item already in the order.

2. Now add a new item to the order (or delete a different existing item).

Stock level adjustments for the item you changed in #1 are not handled by the ajax posts in #2.

The attached patch against 6.x2.x-dev is an attempt to correct this.