icalendar data needs a PRODID (and a DTSTAMP, but apparently GMX only requires the PRODID)

Author: tobixen

tests/test_caldav.py

@@ -846,10 +846,9 @@ def testSchedulingMailboxes(self):
 
     def testIssue397(self):
         cal = self._fixCalendar()
-        cal.save_event(
-            """
-BEGIN:VCALENDAR
+        cal.save_event("""BEGIN:VCALENDAR
 VERSION:2.0
+PRODID:-//PeterB//caldav//en_DK
 BEGIN:VEVENT
 SUMMARY:recurrence with attendee one single item
 DTSTART;TZID=Europe/Zurich:20240101T090000