Fetch units by line item in OrderInventory `verify`

Related to https://github.com/spree/spree-product-assembly/issues/58

Once product assembly is installed Spree should always take into account
line_items when searching for inventory units for, well, a given line
item
Dont depent on db ordering in spec

It was causing random failures probably due to database ordering

Conflicts:
	spec/models/spree/line_item_spec.rb
Consider variant and line item when removing inventory

When removing inventory units we need to filter them per line item as
well. Some orders might have the same product sold as a part and within
a bundle

Fixes #58