[caldav] Improve incidence comparisons
[caldav] Avoid checking Event::hasEndDate()

Event(Event*) initializes mHasEndDate based on other->mHasEndDate
rather than other->dtEnd().isValid() so the hasEndDate() comparison
doesn't work against events loaded from db that have a valid dtEnd.
Merge pull request #22 from nemomobile/improve-compare

Improve incidence comparisons