Clone of https://issues.kolab.org/show_bug.cgi?id=5067

The BusyCal calendaring app apparently uses the COMPLETED property for flagging tasks as complete. This doesn't sync to Kolab and leads to tasks marked as complete in BusyCal not being set as complete in the backend and also tasks with status:complete from the web client still appearing incomplete in BusyCal.

COMPLETED is defined in RFC 5545 Section 3.8.2.1 and holds a UTC timestamp:

COMPLETED:20150527T100222Z

Steps to reproduce and test:

1. Create two tasks "A" and "B" in the web client
2. Sync tasks with BusyCal via CalDAV
3. Mark task "A" as complete in the web client
4. Mark task "B" as complete in BusyCal
5. Sync tasks with BusyCal

Expected results:

• Task "A" appears as complete in BusyCal
• Task "B" appear as complete in the web client