title: Why isn't cxf json marshaing fields with null value
body: <p>cxf version 2.2.7.<br>
  json string only contains not null value, but not contains null value properties, e.g.:</p>
  
  <pre><code>class Bean {String field1;String field2}
  
  Bean bean = new Bean();
  bean.setField1("value1");
  </code></pre>
  
  <p>after marshaing, result json is:</p>
  
  <pre><code>{"field1":"value1"}
  </code></pre>
  
  <p>I expect json to be:</p>
  
  <pre><code>{"field1":"value1","field2":""}
  </code></pre>
  
  <p>Thanks !</p>