Use sort_by to correctly sort return items in the customer return handler
Update customer return handler reimbursement creation

* build reimbursements from the customer return so only non-reimbursed accepted return items get reimbursed
* don't expect a reimbursement for return items for something other than the rma expected per https://github.com/spree/spree/commit/1bcd2792c18d64a83ee4925720f80227a36de30d#diff-e00e5798905985d9f2ce3fb833b17ce4R6
Don't attempt to reimburse customer returns for orders with non-inventory refunds

* These require special handling/calculation, so we don't want to auto-reimburse them
Fix inventory unit serializer to use line item price
Merge branch 'inventory-unit-serializer-specs' of https://github.com/bonobos/spree_wombat
Merge branch 'fix-customer-return-handler-sort' of https://github.com/bonobos/spree_wombat