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
OK. BTW I have the same Java version issue with IDEA too. And as you know SI-Java-DSL project is based on Java 8 from tests perspective - and we live with that already for a long time.