http_caldav:caldav_put() further checks for DTSTART and DTEND

When on subsequent (not the first) real component DTSTART/DTEND have different
value types, fail in the same way, as this is done for the first real component.

DTSTART is the inclusive start, DTEND is the non-inclusive end, so DTSTART cannot be
equal to DTEND. A CUA can unintentionally upload VEVENT where DTSTART is the same
as DTEND: https://gitlab.gnome.org/GNOME/evolution/-/issues/1073 .