title: paper-listbox, when setting the selected to an id (int) will not select the input with the same value
body: <p>I have a Paper-listbox with paper items inside of it.</p>
  
  <p>I set selected equal to the ID of an object, in this case 2. (This is not an index).</p>
  
  <p>It doesnt seem to select the item with that value.</p>
  
  <p>Is there something i am doing wrong with the implementation of list boxes?</p>
  
  <p>here is my sample</p>
  
  <pre><code>&lt;paper-dropdown-menu label="Type" no-animations="true" &gt;
    &lt;paper-listbox id="paperListbox" class="dropdown-content" selected="{{type}}"&gt;
      &lt;paper-item value="1"&gt;Internal&lt;/paper-item&gt;
      &lt;paper-item value="2"&gt;External&lt;/paper-item&gt;
    &lt;/paper-listbox&gt;
  &lt;/paper-dropdown-menu&gt;
  </code></pre>
  
  <p>so, <code>{{type}}</code> will set to 2, but it doesnt select the second item.  How do i resolve this?</p>