Description of Problem:
The tasks table marks tasks that are due tomorrow as being due today if the
current UTC time is past midnight.  E.g. a user at UTC offset -6 will start
seeing tasks due tomorrow as being due today starting around 18:00.
This happens because calendar-model.c:is_overdue() does not handle DATE
values correctly.

Steps to reproduce the problem:
1. Add a task.  Make it due tomorrow; make sure this is a DATE value and
not a DATE-TIME value by not assigning a time to it.
2. Wait until it is midnight in England.
3. Look at the blue tasks!

Extra info:
calendar-model.c:is_overdue() is not checking for dt.is_date being set.  In
this case, dt.tzid will be NULL (DATE values don't have a timezone) and
thus it will look like we are requesting the UTC timezone.  It should be
using model->priv->zone if dt.is_date is set.