title: Convert JsDefined to String
body: <p>I use play json library to process json objects in Scala. In order to get a field <code>id</code> from the json object, I run this code: </p>
  
  <p><code>val id = json \ "id"</code></p>
  
  <p>Then I want to convert <code>id</code> into a String. I tried <code>id.get.toString</code> but instead of <code>doi:10.1186-s13612-016-0045-3</code> I got <code>JsDefined("doi:10.1186-s13612-016-0045-3")</code></p>
  
  <p>How to convert it into a String?</p>