code: protected function buildTableHeader($keys)
      {
          $headers = array();
          foreach ($keys as $key) {
              $headers[$key] = $key === MethodTable::SCALAR
                  ? 'value'
                  : \htmlspecialchars($key);
              if ($this->tableInfo['colClasses'][$key]) {
                  $headers[$key] .= ' '.$this->markupClassname($this->tableInfo['colClasses'][$key]);
              }
          }
          return '<thead>'."\n"
              .'<tr><th>&nbsp;</th>'
                  .($this->tableInfo['haveObjRow'] ? '<th>&nbsp;</th>' : '')
                  .'<th>'.\implode('</th><th scope="col">', $headers).'</th>'
              .'</tr>'."\n"
              .'</thead>'."\n";
      }
docs: Returns table's thead
  
  @param array $keys column header values (keys of array or property names)
  
  @return string