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 INT-3516: Revert SF version to 4.1.1 Add Docs on the matter Fix WS Tests; Revert StubJavaMailSender