title: Gson ignoring map entries with value=null
body: <pre><code>Gson gson = new Gson();
  
  Map&lt;String,Object&gt; map = new HashMap&lt;String, Object&gt;();
  map.put("a", 1);
  map.put("b", null);
  
  System.out.println(gson.toJson(map)); //prints {"a":1}
  </code></pre>
  
  <p>How do I get it to include all entries?</p>