JIRA: https://jira.spring.io/browse/INT-4432
The `MockIntegrationContext.substituteMessageHandlerFor()` method
cannot get the correct output channel of target message handler
since the `outputChannel` property is not initialized.
* Use `getOutputChannel()` method instead of directly accessing
the `outputChannel` property. The `getOutputChannel()` method
guarantees the correct value is retrieved.
* Polishing Copyrights and author name
* Simplify `MockMessageHandlerTests.testHandlerSubstitutionWithOutputChannel()`
and its configuration
**Cherry-pick to 5.0.x**
ed00f62549