Bug 27203: Calculate tax based on ecost if unitprice not set

Bug 27203: Calculate tax based on ecost if unitprice not set

There is code in populate_order_with_prices that is intended to use ecost over unitprice,
it just doens't seem to be working.

Making it more explicit seems to take care of the issue.

To test:
1 - Create a basket
2 - Add an order line, don't set 'Actual cost: ', but use vendor price
and a discount.
3 - Save
4 - Display all columns, 'Actual cost tax inc.' are GST columns aren't set.
5 - Apply batch
6 - Redo 1,2, 3
7 - Display all columns, GST is calculated correctly, unitprice remains 0.

Signed-off-by: Marjorie <marjorie.barry-vila@collecto.ca>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>