Bug 15822: Fix 500 error when searching if items.onloan=0000-00-00

Bug 15822: Fix 500 error when searching if items.onloan=0000-00-00

The calls
  output_pref({ dt => dt_from_string( $date ) })
are wrong and should be replaced with
  output_pref({ str => $date })
For better error handling.

Here we fix the problem of items.onloan when searching

Test plan:
- Set items.onloan=0000-00-00 (UPDATE items SET onloan='0000-00-00')
This can come from old data or bad migration
- Execute a search
=> Without this patch you get
Can't locate object method "ymd" via package "dateonly" (perhaps you forgot to load "dateonly"?) at /home/vagrant/kohaclone/Koha/DateUtils.pm line 225.
=> With this patch you won't get the error

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>