#209 - Added support for Map<String, String> and MultiValueMap<String, S...

See [https://github.com/spring-projects/spring-hateoas/issues/209].

I've added unit tests to cover `@RequestParam` Controller arguments with no name that are Map<String, String> or MultiValueMap<String, String>. The only thing I'm not really happy about in this commit is the untyped cast from `Map` to `Map<String, String>` (and similarly for the other type) but as only these generified types are accepted I don't believe it's necessary to check for other types or add an exception handler around the cast.