[Bug] Unsigned numbers not correctly serialized

[Totalus]

Klaxon does not serialize unsigned value correctly.

Code to reproduce:

val str = json { obj(
    "intVal" to 123,    // Int
    "uintVal" to 123U,  // Unsigned Int
    "longVal" to 123L,  // Long
    "ulongVal" to 123UL // Unsigned Long
)}.toJsonString()

Result:

str = 
{
  "intVal": 123,
  "uintVal": {},
  "longVal": 123,
  "ulongVal": {}
}

Expected result:

str = {
  "intVal": 123,
  "uintVal": 123,
  "longVal": 123,
  "ulongVal": 123
}

Version: com.beust:klaxon:5.5