Change sanity check to actually check sanely

As the comment implies, it was bound to happen to someone, and I guess I was the lucky fool who couldn't read...but without the parentheses operator associativity has the sanity check ensuring `false instanceof FixedColumns`, and that just leads to an unhelpful error trying to [invoke `_fnConstruct` later on](https://github.com/DataTables/FixedColumns/blob/095c43bf6662d9a48aa6fe886527beb759f87fb7/js/dataTables.fixedColumns.js#L275).

Adding in the missing parentheses makes sure the developer gets the appropriate message when they forget `new`.