Bug 21975: get item field in substitutions

Bug 21975: get item field in substitutions

substitutions field stores the item field with 'items.' prefix.
Looks like call of C4::Items::ModItem() works but it will not if ORM object is used in the futur.
So I prefer fix it now and it is needed for main patch.

Test plan :
Run t/db_dependent/Items/AutomaticItemModificationByAge.t

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>