Editing time of event with unprefixed TZID strips time zone (converting it to floating time)

Example:

On a system with a time zone of Europe/London (currently UTC+1, at time of this bug filing) an event with the following:

DTSTART;TZID=Europe/London:20170818T130000
DTEND;TZID=Europe/London:20170818T140000

is correctly displayed in Calendar as 13:00-14:00.

Editing the event in Calendar to change the time to 14:00-15:00 results in:

DTSTART:20170818T140000
DTEND:20170818T150000

I.e. the times have been changed to reflect the input values, but the time zone information has been stripped, resulting in a "floating time" event. Calendar also then immediately displays the floating time event incorrectly as well (treating it as UTC instead of local) - I'll file a separate bug for that.

An event with a TZID containing a registry prefix (for example Evolution) correctly retains the TZID after editing in GNOME Calendar. Example VEVENT fields:

DTSTART;TZID=/freeassociation.sourceforge.net/Europe/London:20170818T090000
DTEND;TZID=/freeassociation.sourceforge.net/Europe/London:20170818T093000

Link to original bug (#786154)