Bug 34305: Account for negative prices on acq details tab

Bug 34305: Account for negative prices on acq details tab

The acquisition detail tab shows the estimated cost while
the order is still pending and the actual price, once received.
At least the tool tip on the table header row says so. This
was not working correctly.

To test:
* Create a basket
* Create a new order line with a negative list price (credit)
* Close the basket
* Verify that the price shows correctly in the acquisition details
  tab on the record
* Receive the order and enter a different negative actual price.
* Reload detail view, verify it still shows the estimated price
* Apply patch, restart_all
* Verify now the correct actual price displays
* Create another basket and order line
* Verify while not yet received, it still shows the estimated price

Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>