unit: validate the behaviour when saving a purged doc (#2480)
unit: validate the behaviour when saving a purged doc (#2480)

* validated the behavior, also make sure we can create the same docID for creating new.
couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios