Using `orderData` applies sorting arrow styling to wrong column

Hi Allan,

I hope you are doing well! I think I might have found a bug:

Description of problem:
When using columnDefs.orderData in DataTables 2.1.8, the sorting arrow icons are not toggled in the "target" column but in the (potentially hidden) column with the "orderData".
The expected result would be that the sorting arrow icons are updated in the sorted-on column, as is the case with DataTables 1.13.11.

Testcase with DataTables 2.x (nightly): https://live.datatables.net/jivalori/1/edit?html,js,output (wrong behavior)
Testcase with DataTables 1.13.11: https://live.datatables.net/biqogobi/1/edit?html,js,output (expected behavior)

Best wishes,
Tobias