Fix using anyjson in fake_notifier

For some reason we were using anyjson.serialize() in our fake_notifier.
That may have been to mirror kombu, but really we need to mirror what we
do when we convert our payloads to primitives, which is using jsonutils.

Closes-Bug: #1403162
Change-Id: I92a4708e0700e88f4b2554aa4e7ad3856f5be1e0