`DTEND` should always be generated for iCloud, even if it's the same as `DTSTART`

[rfc2822]

Even it's invalid. But we never got a report about that before the mapper rewrite, so we should again do it like that.

See bitfireAT/davx5-ose#1859

[sunkup]

This is not a workaround for iCloud. That is, will always do that, not just for iCloud but specificially because of them.