JIRA: https://jira.spring.io/browse/INT-4350
Spring MVC and Spring WebFlux handles `ResponseEntity` via appropriate
`ReturnValue` handlers.
This way all the headers and status code are fully up to end-user.
The body is handled by the appropriate converter/writer as before
* Add `ResponseEntity` handling to the `HttpRequestHandlingMessagingGateway`
and `WebFluxInboundEndpoint`.
The logic mostly compy/pasted from the `ResponseEntityResultHandler`
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