LP#1789442: fix editing due date of hourly loans in web staff

LP#1789442: fix editing due date of hourly loans in web staff

This patch fixes a bug where editing the due date of an hourly
loan would result in the time portion always getting set to 00:00.

To test
-------
[1] Set up some daily and hourly loans in your database.
[2] Apply the patch.
[3] From the items out page, edit the due date for one or
    more hourly loans. Verify that the resulting due date
    is set correctly.
[4] From the items out page, edit the due date for some
    daily loans and verify correctness. In particular,
    verify that the resulting due date is /not/ the
    following day if testing in time zones in the Americas.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>