Don't mutate arguments in SubIterator._next

[andymatuschak]

This method mutates an argument and subsequently references arguments. In JS strict mode, the arguments symbol references a copy of the initial function arguments; it does not reflect mutations made to the arguments in the body of the function (see spec)—so the mutation is inadvertently dropped. I've fixed this issue by using the argument names directly rather than the arguments objects.

[vweevers]

5.0.1

Thanks!