title: Json4s ignoring None fields during seriallization (instead of using 'null')
body: <p>I am having a generic json serialization method that uses <code>json4s</code>. Unfortunately, it is ignoring fields if the value is <code>None</code>. My goal is to have <code>None</code> fields be represented with a <code>null</code> value. I tried by adding custom serializer for None, but still it is not working.</p>
  
  <pre><code>object test extends App {
            class NoneSerializer extends CustomSerializer[Option[_]](format =&gt; (
              {     
                case JNull =&gt; None
              },
              {
                case None =&gt; JNull
  
              }))
  
           implicit val f = DefaultFormats + new NoneSerializer
  
            case class JsonTest(x: String, y: Option[String], z: Option[Int], a: Option[Double], b: Option[Boolean], c:Option[Date], d: Option[Any])
  
            val v = JsonTest("test", None, None,None,None,None,None); 
  println(Serialization.write(v))
  }
  </code></pre>
  
  <p>Result from the above code :</p>
  
  <pre><code>{"x":"test"}
  </code></pre>
  
  <p>I tried <a href="https://stackoverflow.com/questions/16872893/generating-json-string-using-json4s-from-a-list-containing-some-and-none-values">this link</a> and some others, but is is not solving the issue for case classes</p>