title: <th> as slotted node
body: <p>I have defined a shadow template as follows:</p>
  
  <pre><code>  &lt;table id="overviewbox-loadingbox"&gt;
      &lt;thead&gt;
          &lt;tr&gt;
          &lt;slot name="table-header"&gt;&lt;/slot&gt;
          &lt;/tr&gt;
      &lt;/thead&gt;
      &lt;tbody&gt;
          &lt;div id="loadingbox"&gt;&lt;/div&gt;
      &lt;/tbody&gt;
    &lt;/table&gt;
  </code></pre>
  
  <p>The users are supposed to supply their own <code>&lt;th&gt;</code>s by something like:</p>
  
  <pre><code>  &lt;th slot="table-header" data-column_name="description"&gt;describing here&lt;/th&gt;
  </code></pre>
  
  <p>However this doesn't work. As soon as I change <code>&lt;th&gt;</code> to <code>&lt;span&gt;</code>, with all other things unchanged, the slotted node shows up. Is this because there is some undocumented quirks about using <code>&lt;th&gt;</code> and <code>&lt;slot&gt;</code> together? Thanks.</p>