Bug 10483: fix item uniqueness check on order form

Bug 10483: fix item uniqueness check on order form

When syspref "UniqueItemFields" is defined, the item uniqueness is
tested in acquisition by an AJAX call to check_uniqueness.pl.  This
patch fixes an issue where check_uniqueness.pl wasn't looking
at the correct CGI parameters.

Test plan :
- Select an existing item with barcode
- Add "barcode" to "UniqueItemFields" syspref (use space as separator
  for multiple values)
- Set "AcqCreateItem" syspref to "Create an item when placing an order"
- Go to an acquisition basket
- Create a new order from empty
- Enter existing barcode in item form and save
=> You get an alert that barcode already exists and order is not saved
- Enter a non-existing barcode in item form and save
=> Order and item are created

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>