Fix test: #7 broke test by re-adding DTEND to recurring events

- After #7, the length of recurring events is not made null

- This means that all-day recurring events now have a DTEND property
  which is one day after the start day.  This appears to be
  intentional: an all-day recurring event on day N has DTSTART=(N) and
  DTEND=(N+1).  Furthermore, tests with Thunderbird and Google
  Calendar report that the new (post-#7) behavior is imported
  correctly.

- However, #7 has broken the tests, undetected to me.  This fixes the
  test by removing the check which is no longer needed.

Author: rkdarst

tests/test_events.py

@@ -60,7 +60,8 @@ def test_rrule():
         )
     )
     event_str = event.serialize()
-    assert 'DTEND' not in event_str
+    # DTEND exists and is the next day, calendar tools import this
+    # correctly as being a one-day event
     assert 'RRULE:FREQ=YEARLY;UNTIL=20300422T000000' in event_str