title: Button with icon labelled with aria-label still an 'Empty Button' error
body: <p>A button with an <code>aria-label</code>:</p>
  
  <pre><code>&lt;button type="button" class="btn btn-default dropdown-toggle"
    aria-expanded="false"
    aria-label="Sort"&gt;
    &lt;i class="fa fa-arrows"&gt;&lt;/i&gt;
  &lt;/button&gt;
  </code></pre>
  
  <p>is still identified as an accessibility error ('Empty Button') by <a href="http://wave.webaim.org/extension/" rel="noreferrer">WAVE</a>. Any ideas?</p>