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.
```scala
+   * {{{
+   *   import com.paypal.cascade.json._
+   *   case class JsonPatch(op: String, path: String, value: Option[Any])
+   *   case class AnObject(...)
+   *   val jsonStr = JsonUtil.toJson(JsonPatch("add", "/-", Some(AnObject(...)))).get
+   *   val a = JsonUtil.fromJson[JsonPatch](jsonStr).get
+   *   val inner = JsonUtil.convertValue[Option[AnObject]](a.value).get
+   * }}}
```