io.ascii HTML writer ignores fill_values #5354

In [6]: t = Table([[1], [2]], names=('a', 'b'))

In [7]: t
Out[7]: 
<Table length=1>
  a     b  
int64 int64
----- -----
    1     2

In [8]: ascii.write(t, fill_values=('1', 'Hello world'), format='html')
<html>
 <head>
  <meta charset="utf-8"/>
  <meta content="text/html;charset=UTF-8" http-equiv="Content-type"/>
 </head>
 <body>
  <table>
   <thead>
    <tr>
     <th>a</th>
     <th>b</th>
    </tr>
   </thead>
   <tr>
    <td>1</td>
    <td>2</td>
   </tr>
  </table>
 </body>
</html>


In [9]: ascii.write(t, fill_values=('1', 'Hello world'))
a b
"Hello world" 2