Spring Treat Kotlin nullable as non-required

Superseeds #1060

@jhoeller Could you please review the polished version of this pull request, since this is the first time we introduce Kotlin code in our unit tests and since it adds a new Optional<Boolean> MethodParameter#isNullable() method that I would like to validate with you?

Issue: https://jira.spring.io/browse/SPR-14165

Comment From: sdeleuze

I merged a refactored version that reuses MethodParameter#isOptional(), with more tests and polish.