stocknumber not saved properly

if you have a marc field, mapped to items.stocknumber, it's not saved properly in items.stocknumber

Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>
Signed-off-by: Galen Charlton <gmcharlt@gmail.com>

C4/Items.pm

@@ -443,6 +443,7 @@ my %default_values_for_mod_from_marc = (
     replacementpricedate => undef, 
     restricted           => undef, 
     stack                => undef, 
+    stocknumber          => undef, 
     uri                  => undef, 
     wthdrawn             => 0,
 );