title: Incorrect node children when using progressive_render in jsTree
body: <p>I've made a small app that takes in a json string, converts the string to json object which is passed to jsTree.  My tree has nodes that may or may not have children.  My nodes also are of different types, which I have sorted with the sort plugin.  </p>
  
  <p>My original problem is that the tree is loading very slowly (especially in FireFox) and I could see that the problem was in part due to the sort plugin.  One solution that I found while roaming the interwebs was to use jsTree JSON plugin's "progressive_render" option.  This dramatically increased the speed of rendering my tree.  However, the nodes that once did not have any children (and thus did not have a open/close arrow), now have a open/close arrow as if they had children.  If I click on the arrow to open the node, my application crashes.  How can I use progressive_render (for speed), but have my nodes correctly recognize if they have children or not?</p>
  
  <pre><code>  function createNode(node){
      node = {
          "data" : node.name ? node.name : "",
          "attr" : {rel:node.type ? node.type : ""},
          "correct_state" : "closed",
          "children" : node.children ? node.children : []
      };
      return node;
  }
  </code></pre>
  
  <p>Thanks,
  Sean</p>