Reduce round-trips on PUT for meetings
When uploading meetings/tasks, invitations for which will be sent by the server, calendar-factory sends PUT … Prefer: return=minimal. The server adds to the attendees a SCHEDULE-STATUS= parameter, so it never returns ETag, but a Schedule-Tag header.
If Evolution is not going to show the Schedule-Status parameter, then use the Schedule-Tag to verify that nothing has changed.
Otherwise send Prefer: return=representation to avoid subsequent GET