use get_field so index matches properly

We noticed our bSearchable_n index was not matching up to get_db_fields()[index]. This is because get_db_fields() returns ALL fields for a model not just the ones in your data.  Using get_field() solved this issue for us however it still chokes if you have curly braces in your DataTablesView fields dictionary:
fields = {
        'user': '{user__first_name} {user__last_name}',
}

had to become 

fields = {
        'user': 'user__username',
}