Handle orders that are already partially returned

This change allows for automatic processing even if an order is already
partially returned. If, for example, the payload indicates that we
should create a customer return for items a, b and c but item a was
already returned, this change allows for automatic processing of b and
c. Previously, this scenario would have resulted in a Wombat error.
Explicitly handle an empty request
Merge pull request #33 from bonobos/handle-partial-returns

Handle orders that are already partially returned