Only reload shipment items on item adjustment

The original issue fixed by
https://github.com/spree/spree/commit/a4b209c7288198124a735d99203102d86be6a935
only affected shipment calculations. Removing the reload for LineItems
allows them, and more important their descendants to be eager loaded.
Pass item to Adjustment#update to avoid N+1
Merge pull request #1 from bloomcrush/fix-update-item-attributes-performance

Fix update item adjustment update performance