title: Backbone Model save - this.model.get('id') undefined
body: <p>Using this:</p>
  
  <pre><code>        this.model.save(null, {
              success: function(model) {
                  app.router.navigate('items/' + model.get('id'));
                  this.render();
              }
          });
  </code></pre>
  
  <p>Has anyone got any ideas why model.get('id') would be undefined, yet model.id has the new id from the server? </p>