fix table border rounding for single column

Created by: ttsu

If a bordered table has a single column for the first row, the top-left corner does not get rounded properly. This can be fixed with an :only-child selector.

<table class="table table-bordered">
   <thead>
     <tr>
        <th colspan="2">People</th>
     </tr>
     <tr>
        <th>First Name</th>
        <th>Last Name</th>
     </tr>
   </thead>
   <tbody>
      <tr>
         <td>Tim</td>
         <td>Tsu</td>
      </tr>
      <tr>
         <td>Joe</td>
         <td>Schmoe</td>
      </tr>
    </tbody>
</table>

People
First Name	Last Name
Tim	Tsu
Joe	Schmoe