Fix ruby syntax error

cherry-picked from 7ab494d7cba7e564672df201cc6329901b753e2b in spree master
Return item cancellation

Creating a new return item should cancel any other return items with the same inventory unit.
This ensures an inventory unit can only be active in a single RMA at a time.
This allows history to be preserved but still allow new RMAs to be created to replace old ones.

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

cherry-picked from 108e01331a35ec4a59e7253d55d3cce3339d0e50 in spree master