Bug 20485: Don't account for items timestamps when items not included

Bug 20485: Don't account for items timestamps when items not included

If combining "--dont_export_items" and "--date" options
in export_records.pl, don't include items modification/deletion
timestamp in biblio query conditions.

How to test:
1) Delete or modify a biblio item.
2) Run export_records.pl with "--dont_export_items" and "--date"
   options with a date value before item was deleted or modified,
   but after the biblio was modified.
3) The biblio for which an item was changed should not be included in
   the export.

Sponsored-by: Gothenburg University Library
Signed-off-by: Maryse Simard <maryse.simard@inlibro.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>