expose convertValue in JsonUtil, clean up tests

This also makes it so that all of the JsonUtil tests use implicit wrapper methods instead of the JsonUtil object methods, so that we're always testing through those instead of having a basic little test at the end that does so.
What happens instead is that the `List[Int]` gets successfully converted to a `List[String]` because Jackson knows that Ints have a String representation. This is a known quirk, and we test for it too.