title: bootstrap collapse does not animate when opened programmatically
body: <p>I have a site with a <a href="https://getbootstrap.com/docs/3.3/javascript/#collapse" rel="nofollow noreferrer">bootstrap collapse</a> in it.  I am trying to make it open programatically when the user selects a certain radio button, its working but the collapse does not animate in it just pops in.<br>
  The code I'm using is:</p>
  
  <pre><code>$("#RadioBtn").on('change', function(){
      $("#RadioCollapse").addClass('in');
  })
  </code></pre>
  
  <p>I suspect I may have to do something involving the <code>collapsing</code> class, but I try applying that class to the element and it didn't animate either. </p>