Bug 17502: Add type check to output_pref

Bug 17502: Add type check to output_pref

This patch makes the following changes:
[1] In Koha/DateUtils.pm, sub output_pref:
    Add a test if $dt is really a DateTime before calling method ymd.
    Preventing a crash like:
    Can't locate object method "ymd" via package "dateonly".
    See also BZ 17502/15822.
[2] Adds a few unit tests in t/DateUtils.t.

Test plan:
[1] Run the adjusted unit test t/DateUtils.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>