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JIRA: https://jira.spring.io/browse/INT-4284 Add `INFO` into the `MessageBuilder#copyHeadersIfAbsent()` when end-user tries to populate headers which are `readOnly` We can't throw exception on the matter since can modify `readOnlyHeaders` and that would force end-user to add `header-filter` logic to the application. * Document `readOnly` headers in the `message.adoc` **Cherry-pick 4.3.x**