title: Jquery tokenInput not prePopulating
body: <p>Hi I'm following <a href="http://railscasts.com/episodes/258-token-fields" rel="nofollow">this tutorial</a> but prepopulating the field is not working while editing my model..</p>
  
  <p>Note that text_field category_tokens is not pre-populated with categories while editing.</p>
  
  <p>Here are code snippets, the HTML output of the text_field shows the values.</p>
  
  <pre><code>  &lt;input id="product_category_tokens" type="text" size="30" name="product[category_tokens]" data-pre="[{"created_at":"2010-09-13T03:33:17Z","description":"","id":x,"name":"Kitchen &amp; Dining ","parent_id":xx,"permalink":"kitchen-dining","updated_at":"2011-01-05T11:17:10Z"}]" style="display: none;"&gt;
  
    application.js
      $(function() {
        $("#product_category_tokens").tokenInput("/categories.json", {
            crossDomain: false,
            prePopulate: $('#product_category_tokens').data('pre'),
            preventDuplicates: true
        });
      });
  
      &lt;%= f.text_field :category_tokens, "data-pre" =&gt; @product.categories.map(&amp;:attributes).to_json %&gt;
  </code></pre>