Only count <th>s on the current <tr>

One-line bug-fix!

To determine the column to sort, algorithm was summing 'colspan' values of all `<th>`s in the table, up to the one with the same index as the clicked `<th>`.

However, it should only have been examining the 'colspan' values of the `<th>`s in the `<tr>` of the clicked `<th>`.

Gets the first ancestor `<tr>` (`closest("tr")`) and examines its elements instead of the whole table.