title: jsdom 9.1+ does not set document.activeElement when focusing a node
body: <p>I'm using jsdom with enzyme+mocha+chai to test the behavior of a React component. The component has a method to focus a DOM node (using the usual <code>node.focus()</code>) and I want to test the node is actually focused when it's called.</p>
  
  <p>To know which node is focused, I compare <code>document.activeElement</code> to the node I expect to be focused.</p>
  
  <p>However, after upgrading to jsdom 9.1+, <code>document.activeElement</code> seems always be <code>HTMLBodyElement</code>, even after calling the node's <code>focus()</code> method. </p>
  
  <p>With jsdom 9.0 the tests run fine.</p>
  
  <p>I read jsdom 9.1+ <a href="https://github.com/tmpvar/jsdom/blob/master/Changelog.md#910" rel="noreferrer">contains some changes</a> related to the focus event, yet I couldn't understand how make <code>document.activeElement</code> behave as expected. Any help?</p>