Bug 20455: Fix sort patrons by date expired

Bug 20455: Fix sort patrons by date expired

We may need to apply this change for all other occurrences in our
codebase.

Test plan:
- Make sure sample patron data is loaded and you have a patron without a
date expired defined
- Make an empty search for all patrons or another search that gives you
a good amount of results
- Try to sort on the date expired column

=> Without this patch you get
Odd number of elements in anonymous hash
at /home/vagrant/kohaclone/C4/Utils/DataTables/Members.pm line 189.
output_pref is called with 'dateonly' (ref SCALAR), not a DateTime
object

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>