title: XStream useAttributeFor not working
body: <p>XStream refuses to add the "resultState" field as attribute in the output xml below (see screenshot), yet it does it correctly for the "order" and "name" fields.  Any idea why?</p>
  
  <p>Code:</p>
  
  <pre><code>xstream.useAttributeFor(Result.class, "name");
  xstream.useAttributeFor(Result.class, "order");
  xstream.useAttributeFor(Result.class, "resultState");
  </code></pre>
  
  <p>With: - Result.class, the parent class of ScenarioResult, TestCaseResult, PhaseResult and TaskResult shown in the xml output below.
   - "resultState", a field of Result.class and of type ResultState (it's an enum type) </p>
  
  <p>XML output (please see screenshot attached as plain xml text doesn't show properly on the post):
  <img src="https://i.stack.imgur.com/XqoFN.jpg" alt="enter image description here"></p>