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; I realized we needed to bump to java 8 for this, but didn't expect to see java7 syntax.

I suppose it's ok.

That said, the mixed java versions causes big problems with the gradle `eclipse` task; the project metadata gets screwed up and I have to manually reconfigure every project (java version).

I think we'll have to comment out the `Optional` test for now (and fix these Java7 usages).