JIRA: https://jira.spring.io/browse/INT-2480
* Introduce `headers-function` option into the `aggregator` for merging
and computing headers for the output message based on the completed
group
* Implement a `DefaultAggregateHeadersFunction` and use it in the
`AbstractAggregatingMessageGroupProcessor` for default behavior with
possible injection for any other implementation
* Add `DelegatingMessageGroupProcessor` to wrap any other
`MessageGroupProcessor` implementations with possible usage of the
`headersFunction` if result is not a `Message` or `MessageBuilder`
* Make `AbstractCorrelatingMessageHandler.getOutputProcessor()` as
`public` rto give access to this option from the `AggregatorSpec` to
be able to inject a `headersFunction` in Java DSL configuration
* Add `AbstractIntegrationMessageBuilder.getHeader()` to get access to
some underlying header avoiding extra `Map` in case of `getHeaders()`
* Change a logic in the `AbstractMessageProducingHandler.produceOutput()`
to consult a `reply` for the `replyChannel` as well `routingSlip` header
if the `reply` is a `Message` or `MessageBuilder`
* Introduce a `AbstractMessageProducingHandler.messageBuilderForReply()`
and use it in `AbstractMessageSplitter` to avoid duplication
* Validate a new functionality in tests
* Fix `FileOutboundGatewayParserTests` to rely on the `TemporaryFolder`
to clean up test files after using
* JavaDocs for `DefaultAggregateHeadersFunction`
* Some `router.adoc` polishing
* Fix link to Reactor in the `router.adoc`
* Add docs for new `Function<MessageGroup, Map<String, Object>>` strategy
* Doc polishing.
5a1846cfe5