title: Affix is not working in Bootstrap 4 (alpha)
body: <p>According to Bootstrap 3 docs I have added following attributes to a navbar:</p>
  
  <pre><code>&lt;nav class="navbar no-margin-bottom" data-spy="affix" data-offset-top="90" &gt;
  ...
  &lt;/nav&gt;
  </code></pre>
  
  <p>After scrolling down the page Bootstrap 4 is not adding class to navbar which is affix. Can anyone tell me how to solve this problem? Bootstrap.js and jQuery.js are working.</p>