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.
more tests, suggested changes, and a caveat!
remove leftover forAllNoShrink
Merge pull request #55 from devmage/feature/expose-jackson-convertvalue

expose convertValue in JsonUtil, clean up tests