Bug 12619 - Shipment date gets lost on finishing and/or editing the invoice

Bug 12619 - Shipment date gets lost on finishing and/or editing the invoice

To reproduce:
- Create a new shipment, make sure to add a shipment date
- Receive or not receive orders
- Finish receiving with the button at the bottom of the page
- Verify that shipment date is now empty

To test:
- reproduce the aforementioned issue
- apply patch
- confirm that the issue is no longer rerpoductible (= shipment date is
not getting lost any longer), and that there are no apparent regresssions
of any kind involving invoice shipment date entering and/or editing
- sign off

Signed-off-by: Aleisha <aleishaamohia@hotmail.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes tests and QA script, fixes the issues, no problems found.

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