title: apache wink server stub return null,the rest client throw exception
body: <h1>Server stub</h1>
  
  <pre><code>  @GET
      @Path("/user/{id}")
      @Produces(MediaType.APPLICATION_JSON)
      public User getUser(@PathParam("id") String id){
          User user =  myService.getUserById(id);
          if (user!= null){
              return user;
          }
          return null;
      }
  </code></pre>
  
  <p>==============================
  Client stub</p>
  
  <pre><code>User response = WinkRestClient.resource(path).contentType(MediaType.APPLICATION_JSON).accept(MediaType.APPLICATION_JSON).get(User.class);
  </code></pre>
  
  <p>throw exception:</p>
  
  <pre><code>Exception in thread "main" java.lang.RuntimeException: A javax.ws.rs.ext.MessageBodyReader implementation was not found for class com.test.entity.User type and application/octet-stream media type.  Verify that all entity providers are correctly registered.  Add a custom javax.ws.rs.ext.MessageBodyReader provider to handle the type and media type if a JAX-RS entity provider does not currently exist.
      at org.apache.wink.client.internal.handlers.ClientResponseImpl.readEntity(ClientResponseImpl.java:122)
      at org.apache.wink.client.internal.handlers.ClientResponseImpl.getEntity(ClientResponseImpl.java:65)
      at org.apache.wink.client.internal.handlers.ClientResponseImpl.getEntity(ClientResponseImpl.java:52)
      at org.apache.wink.client.internal.ResourceImpl.invoke(ResourceImpl.java:196)
      at org.apache.wink.client.internal.ResourceImpl.get(ResourceImpl.java:303)
      at com.wolianw.sale.bl.ClResourceTest.main(ClResourceTest.java:14)
  </code></pre>
  
  <p>when server stub return null ,the client throw this exception</p>
  
  <p>what can I do,let the client don't throw exception,let response [User = null]</p>