title: JSONObject's method toString() is always null
body: <p>I'm trying to test a method with a <code>JSONObject</code> parameter, but when I create an instance of <code>JSONObject</code> and put some values, the <code>toString()</code> value returned from the instance is null. I'm expecting a JSON string. See code below.</p>
  
  <pre><code>val jsonObject = JSONObject()
  jsonObject.put("message", "hello")
  println(jsonObject.toString()) // actual: null, expected: {"message":"hello"}
  </code></pre>