Iterate through groups of inventory units rather than each inventory unit one at a time in Shipment#to_package

Imagine in the case where there are 100 inventory units for the shipment. In this situation, the old code was calling this code 100 times:

package.add inventory_unit.line_item, 1, inventory_unit.state_name

Conflicts:
	core/app/models/spree/shipment.rb