INT-651: Add `Iterator` support for `Splitter`

JIRA: https://jira.spring.io/browse/INT-651
`s/we aren't interested in/we don't have access to/`

Also, the header is not 0, it's `null` - and it looks like we have a problem there - in SI 3.0, setting a header to null removed it; looks like that functionality was broken when we moved to o-s-messaging.