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

JIRA: https://jira.spring.io/browse/INT-651
Strange.. I thought that I had changed it to the `0`, since it is a default value, when `sequenceSize` is `null` - `IntegrationMessageHeaderAccessor`:
```
public Integer getSequenceSize() {
	Integer sequenceSize = this.getHeader(SEQUENCE_SIZE, Integer.class);
	return (sequenceSize != null ? sequenceSize : 0);
}
```

Thanks, will be pushed soon with all mentioned concerns