1099945 - use correct serializer when publishing http events

Previously, the default json serializer was used when publishing events. If an
event had a bson ObjectId in it, the publish would fail due to a json
serialization error.

Instead, use the serializer from json_util. This also fixes the amqp and mail
notifiers, which had the same bug.
Merge pull request #983 from beav/bz109945

1099945 - use correct serializer when publishing events