Bug 20487: AddReturn should clear items.onloan for unissued items

Bug 20487: AddReturn should clear items.onloan for unissued items

If an item is no longer issued but somehow still has a date in the onloan
column, checking it in should clear that date.
Adding a ModItem call in the NotIssued section.

Test plan:
[1] Run t/db_dependent/Circulation.t
[2] Bonus: Checkout item, delete issue from table, checkin. Verify that
    items.onloan has been cleared.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Charles Farmer <charles.farmer@inLibro.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>