title: CSSTransition from react-transition-group does not exit
body: <p>Trying to get used CSSTransiction component. I need to show a panel which is animated by using css-classes. The first class defines the primary location, the second defines position when the panel is opened (top: 0).</p>
  
  <p>Appearing is working, but exiting does not. onEntered fires, onExied does not.</p>
  
  <p>There is lack of documentation for CSSTransition component, I tried different combinations, but without success. Any ideas?</p>
  
  <p>CSS:</p>
  
  <pre><code>.getSecondLevelPanel{
      position: fixed;
      left: 450px;
      top: -100%;
      height: 100%;
      width: 400px;
      transition: all 250ms ease-out;
  }
  
  .getSecondLevelPanelOpened {
      top: 0;
  }  
  </code></pre>
  
  <p>React component: </p>
  
  <pre><code>import * as React from 'react';
  import { CSSTransition } from 'react-transition-group';
  
  export interface Props {
      isVisible: Boolean;
      children: React.ReactChild;
      onClose: () =&gt; void;
  }
  
  const SecondLevelPanel = (props: Props) =&gt; {
    return (
      &lt;CSSTransition
          timeout={{
              enter: 0,
              exit: 500
          }}
          in={props.isVisible}
          appear={true}
          enter={true}
          exit={true}
          classNames={{
              appear: 'getSecondLevelPanel',
              enterDone: 'getSecondLevelPanel getSecondLevelPanelOpened',
              exit: 'getSecondLevelPanel'
          }}
          unmountOnExit={true}
          onEntered={() =&gt; {
              alert('entered');
          }}
          onExited={() =&gt; {
              alert('exited');
          }}
      &gt;
          &lt;div&gt;
              &lt;a onClick={() =&gt; props.onClose()}&gt;Close&lt;/a&gt;
              {props.children}
          &lt;/div&gt;
      &lt;/CSSTransition&gt;
    );
  };
  
  export default SecondLevelPanel;
  </code></pre>