INT-3516: Allow `Optional<>` in POJO method args

JIRA: https://jira.spring.io/browse/INT-3516

* Add `Optional<>` support for `@Header` in the `MessagingMethodInvokerHelper`
* Add `Optional<>` test-case
* Change `sourceCompatibility` for test to the Java 8
Right, but we can't live with a bad gradle `eclipse` task - don't do anything yet, I am trying again with the newest version of STS.