Bug 19039: Results of virtual shelves (lists) not sortable by date

Bug 19039: Results of virtual shelves (lists) not sortable by date

In the staff client, when viewing the content of a list, it can be sorted by 'title', 'author' or 'call number' but not by 'date added'.

Test plan:
1) In the staff client, view a list containing several items
2) Notice that you can't sort by 'date added'
3) Apply the patch
4) Repeat step 1
5) The list can now be sorted by 'date added'

Signed-off-by: Dilan Johnpullé <dilan@calyx.net.au>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>