[Calendar] Preserve the organizer/attendee SCHEDULE-STATUS parameter
Per https://tools.ietf.org/html/rfc6638#section-7.3, when a CalDAV client uploads an event with attendees and the server sends iMIP messages to the participants, it includes a SCHEDULE-STATUS parameter to each ATTENDEE in the iCalendar object of the organizer. “Clients SHOULD NOT change or remove this parameter if it was provided by the server.” The latter means, that once the CalDAV client knows the SCHEDULE-STATUS for an attendee, on subsequent PUTs, this status shall be sent in the iCalendar object. Evolution strips SCHEDULE-STATUS.
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