Do not assert in dav resource calling ResourceBase::collectionsRetrieved from custom job
Review Request #129346 - Created Nov. 6, 2016 and discarded
ResourceBase::collectionsRetrieved assumes it must be called from either a Sync or SyncAll task.
The dav resource however does this from a Custom task.
Therefore the dav resource always crashes (asserts) in a debug build.
This patch simply also allows the Custom Task.