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**
Conflicts:
spring-integration-core/src/test/java/org/springframework/integration/gateway/GatewayInterfaceTests.java
Resolved.
37cf8336de