JIRA: https://jira.spring.io/browse/INT-3979
Previously we had a to instantiate all messages from the `MessageBuilder`
if we would like to return messages as splitted items.
For example to populate some item-specific headers.
From other hand the `MessageBuilder` as an item incorrectly remained as a `payload`.
* Add condition logic to the `AbstractMessageSplitter` to properly handle `AbstractIntegrationMessageBuilder`
and don't create an extra `Message`
* Apply the same logic to the `AbstractAggregatingMessageGroupProcessor`
(We may consider to backport it later)
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