Bug 18885: Specify due date behavior according to on-site checkbox

Bug 18885: Specify due date behavior according to on-site checkbox

When a on-site checkout is performed a date is automatically added but
if you unchecked the date remains. Even with OnSiteCheckoutAutoCheck
syspref

Test plan:
1) Do an on-site checkout by clicking on checkbox and see date added
2) Unchecked and see that the date remains on input
3) Do another test with OnSiteCheckoutAutoCheck enabled
4) Apply this patch and repeat actions

Normally now its works correctly

Signed-off-by: tuxayo <victor@tuxayo.net>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>