Add failing spec to expose bug
Add failing spec to expose bug

The `Spree::ReturnAuthorization#amount` method currently calls:
  `return_item.sum(:amount)`
This will raise a method missing error because the association is a
`has_many`, so we need to call `return_items.sum(:amount)`.

pervino/solidus,pervino/solidus,pervino/solidus,pervino/solidus