Packer now tests whether it should track inventory.

Spree::Stock::Packer.product_assembly_package tests whether inventory is being tracked,
as Spree's #default_package does. If inventory is not being tracked, the part or variant
is added with an :on_hand status. Specs included. Without this, you won't be able to ship backordered items using spree-product-assembly, no matter what your inventory tracking and shipment config options are.