title: Genson serialization for maps with complex keys
body: <p>One of the feauture listed on Genson home page is 
  "Serialization and Deserialization of maps with complex keys".</p>
  
  <p>While I'm trying to serialize a map with Keys as complex java objects into json string, and then deserialize them back to java Map. The deserialized map keys are always strings. 
  Can someone please help me on how to use genson for such complex key map serialization and deserialization?</p>
  
  <p>Here is my code</p>
  
  <pre><code>    Genson genson = new GensonBuilder().useClassMetadata(true).useRuntimeType(true).create();
      VO vo = new VO();
      Key key = new Key(18314212, new Timestamp(System.currentTimeMillis()),new Timestamp(System.currentTimeMillis()));
      vo.setEndTime(new Timestamp(System.currentTimeMillis()));
      vo.setStartTime(new Timestamp(System.currentTimeMillis()));
      vo.setItemID(18314212);
      vo.setKey(key);
      Map&lt;Object, Object&gt; map = new HashMap&lt;Object, Object&gt;();
      map.put(key, vo);
      String json  = genson.serialize(map);
      System.out.println(json); //the json map key does not have @Class attribute 
      Map jsonMap =  genson.deserialize(json, Map.class);
      System.out.println(jsonMap);
  </code></pre>