JIRA: https://jira.spring.io/browse/INT-4537Fixesspring-projects/spring-integration#2582
* Rename `ReactiveStreamsConsumer.messageHandler` property to the
`handler` for consistency with other `IntegrationConsumer` s
* Do not wrap `Subscriber` into the `MessageHandler` if that one is
already a `MessageHandler`
* Fix `MockIntegrationContext` for the logic around `ReactiveStreamsConsumer`
where it is not enough just replace a `handler`, but we also need to do
that with the `subscriber`.
Luckily the `MockMessageHandler` is also a Reactive `Subscriber`
* Clean up `MockIntegrationContext.beans` in the end of `resetBeans()`
* Improve `testing.adoc`
**Cherry-pick to 5.0.x**
2df71fba30