title: Persist java.util.Properties as serialized object using JDO/DataNucleus
body: <p>The Google AppEngine docs say that I can persist serializable objects using JDO like so</p>
  
  <pre><code>import javax.jdo.annotations.Persistent;
  import DownloadableFile;
  
  // ...
  @Persistent(serialized = "true")
  private DownloadableFile file;
  </code></pre>
  
  <p>but if I use it with Properties</p>
  
  <pre><code>@Persistent(serialized="true")
  private Properties initProps;
  </code></pre>
  
  <p>I get</p>
  
  <blockquote>
    <p>DataNucleus Enhancer (version 1.1.0) : Enhancement of classes</p>
    
    <p>Field "initProps" in class "ServletRegistration" has been defined as a Map but the key type is not specified!</p>
  </blockquote>
  
  <p>Can I fix that with additional annotations?</p>