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.
Add line_item to package if not tracking inventory
Merge pull request #65 from kianw/2-1-stable

Packer now tests whether it should track inventory.