Bug 7451 - PrepareItemrecordDisplay missing $subfield_data{id} which breaks AJAX...

Bug 7451 - PrepareItemrecordDisplay missing $subfield_data{id} which breaks AJAX value_builders

In current version of code, it gets initialized too late, so it produces
unitialized warnings for hidden fields (which is non-fatal) and breaks
generated JavaScript for AJAX value_builders (which is fatal)

This bug was introduced in ticket 6106 which is modification of 5955
which didn't have this particular problem.

Since then, this code moved to C4::Items because of de-nesting patches.

Test scenario:
1. define 952$i plugin: stocknumberam123.pl (this is ajax value_builder)
2. go to one of following pages and verify that plugin works:
	acqui/addorderiso2709.pl
	acqui/neworderempty.pl
	acqui/orderreceive.pl
	serials/serials-edit.pl

For stocknumberam123.pl plugin to work, you have to have at least
one item with stocknumber in format which this plugin expects:
capital letters, space, some number

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works nicely, thanks for the great test plan.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>