Bug 12891: NewOrder does not return ordernumber if ordernumber is defined

Bug 12891: NewOrder does not return ordernumber if ordernumber is defined

The behavior is quite weird, but
  $schema->resultset('Table')->create($data)->id
does not return the id inserted if $data contains the key.

To be more clear, in this case
  $schema->resultset('Aqorder')->create($new_order)->id
returns an empty string because $new_order->{ordernumber} is an empty
string!

This was not caught by the unit tests, I added one.

Test plan:
- AcqCreateItem set to ordering
- Create an order with items and verify items are correctly linked to the
  order.

Signed-off-by: Dobrica Pavlinusic <dpavlin@rot13.org>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Confirmed that without the patch the created item is not linked to the
order (entry in aqorders_items). With the patch, it works as expected.
Passes tests and Koha QA script.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>