Only restock on_hand inventory units

Example: Spree restocks items once you cancel an order. However it
shouldn't add stock (restock) items from backordered inventory units.

Conflicts:
	core/spec/models/spree/shipment_spec.rb
Restock items but dont process backorders for canceled orders

Fixes possible issue where canceling an order with backordered inventory
units would make other orders inventory transition from backordered to
on_hand. e.g.

    - create two separate orders of the same item at the same stock location
    - both orders will have shipment state of backorder
    - cancel one of the orders
    - the second order inventory will transition from backorder to ready

Now the second order inventory remains backorder
[backend] nil check for shipping_method in orders/_shipment

fixes #4025

Conflicts:
	backend/app/views/spree/admin/orders/_shipment.html.erb