Bug 37345: Only toggle_onsite_checkout() if OnSiteCheckoutAutoCheck is enabled

Bug 37345: Only toggle_onsite_checkout() if OnSiteCheckoutAutoCheck is enabled

To test:
1. Find an item to checkout and a patron to check out to.
2. Set a due manually: "Specify due date (MM/DD/YYYY):"
3. Before checking out click the "Remember for session:" checkbox.
4. Check the item out.
5. The specific due date is not retained.
6. APPLY PATCH
7. Try 1 -4 again, now the date should be sticking.
8. Turn on the OnSiteCheckouts system pref and make sure it still works
9. Turn on the  OnSiteCheckoutAutoCheck system pref and make sure the on-site checkbox is still checked after doing an on-site checkout.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>