Bug 19388: FIX display item status if item is checked out

Bug 19388: FIX display item status if item is checked out

In course reserves, if an item which was added to a course is checked
out, it is not possible to show details for that course on OPAC.
The error says: "Template process failed: undef error - The method
onsite_checkout is not covered by tests! at
/home/koha/src/C4/Templates.pm line 121."

onsite_checkout is an attribute of Koha::Checkout, not Koha::Item

Test plan:
Create a course with 2 items that are checked out (standard and on-site)
At the OPAC, add them to your cart and confirm the status of these 2
items is correct
Confirm that on the detail page of the bib record as well as the detail
of the course.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>