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
True. See https://github.com/spring-projects/spring-integration/pull/1276/files#diff-c197962302397baf3a4cc36463dce5eaR71 and https://github.com/spring-projects/spring-integration/pull/1276/files#diff-ff2e3d616fef4dc3dcd8f6e67845ccbcR524
We talked with you before that from here we start use Java 8 from tests.