Create only the column from the first <tr>

This valid markup, which repeat a `<tbody>` instead of a `<tr>`
```html
<table ng-table='tableParam'>
  <tbody ng-repeat='item in data'>
    <tr>
      <td>blabla</td>
      <td>blabla</td>
    </tr>
    <tr>
      <td>blabla</td>
      <td>blabla</td>
    </tr>
  </tbody>
</table>
```

will actually produce 4 columns instead of 2 as the current selector for column creation is getting all the `<td>` element of the template. So here is a simple modification so that only the first row determines the number of column.