JIRA: https://jira.spring.io/browse/INT-4457
When an `IntegrationFlow` is reply-based (we expect a reply in the
beginning) and `log()` (or `wireTap()`) is used in the end, we are
forced to add an empty `bridge()` in the end to ensure a `replyChannel`
header routing
* Fix `IntegrationFlowDefinition` to add `enrichHeaders()` in the end
to populate a `nullChannel` as a `replyChannel` header if that is missed
in the request message headers.
This way we cover both use-cases when we expect reply from the flow and
when it is used as a one-way scenario
* Improve a `HeaderEnricher` do not create a new `Message` if there are
no new headers to add/remove
* Remove a note from the `dsl.adoc` about now redundant `bridge()` after
`log()`
* Resolve TODO in the `.handle()` paragraph
* Restore anonymous class in the MessageHistoryIntegrationTests:
Mockito can't mock/spy lambdas because they are `final` classes
* Introduce `IntegrationFlowDefinition.logAndReply()` operator
* Fix logging message
Doc Polishing
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