icalcomponent_set_due now updates the TZID parameter of the due property, similar to how icalcomponent_set_dtstart and icalcomponent_set_dtend behave. It was a bit strange having set_dtstart and set_dtend update the TZID while set_due didn't.
Also added an equivalent change for icalcomponent_get_due. Before it wouldn't try to read the TZID and set zone on the time, which differed from icalcomponent_get_dtstart/dtend.
Superseded by https://github.com/libical/libical/pull/162