title: Serialize to json default value of Map type as a part of case class
body: <p>I have a case class that includes a default value of Map[String, String].</p>
  <p>something like that:</p>
  <pre><code>case class Car(brand: String, color: String, extraData: Map[String, String] = Map.empty[String, String])
  </code></pre>
  <p>I tried to create a custom json format for that:</p>
  <pre><code>implicit object carJsonFormat extends RootJsonFormat[Props]  {
      override def write(obj: Props): JsValue = ???
  
      override def read(json: JsValue): Props = ???
    }
  </code></pre>
  <p>but had difficulty with dealing with the Map type when I tried to override write and read.</p>
  <p>Does anyone have an idea for an implementation?</p>