Don't be so optimistic about collection detection.

For clients that can't create collections with MK* requests and that don't
use GET requests, is_leaf gives False for empty collections only reached
through a PROPFIND request. As Radicale can't create a collection for
each type of request, we have to strip the last part of path for paths ending
with .ics and .vcf, as they can be represent items and not collections.