title: ListPicker bind value always set to index and not valueField
body: <p>I have this simple array categories being displayed on the html template. The issue is that when I select the item on the list, it sets the <code>book.category</code> to the index of the item on the ListPicker and not the specified valueField. I am not sure if this is the default behavior or I am missing something.</p>
  
  <p>Array:</p>
  
  <pre class="lang-js prettyprint-override"><code>public categories = [
          {id: 6, name: "Horror"},
          {id: 8, name: "Biography"},
          {id: 5, name: "Drama"},
          {id: 7, name: "Comedy"},
          {id: 2, name: "Fiction"}
      ];
  </code></pre>
  
  <p>HTML(Angular):</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;ListPicker [items]="categories" textField="name" valueField="id" 
         [(ngModel)]="book.category"&gt;&lt;/ListPicker&gt;
  </code></pre>