Serialize unsigned integers to JSON

Currently, unsigned integer types are serialized as empty object `{}`, I think Klaxon should serialize them as numbers.
https://kotlinlang.org/docs/unsigned-integer-types.html#unsigned-integers-literals


#### Actual
```kt
println(JsonObject(mapOf("uint" to 1u)).toJsonString()) // {"uint":{}}
```

#### Expected
```kt
println(JsonObject(mapOf("uint" to 1u)).toJsonString()) // {"uint":1}
```