title: When item added, DetailsList doesn't update
body: <p>I'm using <code>office-ui-fabric-react</code> library. Current version that I'm on is 7.17.0.</p>
  
  <p>My list is super simple:</p>
  
  <pre><code>&lt;DetailsList
      columns={this.state.columns}
      items={this.state.items}
      compact={true}
      selectionMode={SelectionMode.none}
      enableUpdateAnimations={true}
  /&gt;
  </code></pre>
  
  <p>When I change the items in a separate method (marked async) and do <code>this.setState({items: someNewItems})</code>, the list doesn't get re-rendered. I'm curious is this a known issue, or is the <code>DetailsList</code> needs to be handled in a special way?</p>
  
  <p>To clarify, after executing <code>this.setState</code> nothing changes, but if I resize the page, the new elements appear suddenly.</p>