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.

Conflicts:
	app/models/spree/stock/packer_decorator.rb
Package#add takes a line_item in first argument now

Adapt changes from #65 to current spree edge