Convert NodeLists to arrays via a loop sted slice

Apparently, Firefox's `Array.prototype.splice` sets `length`. This fails without
a setter. We cannot add a `length` setter, as that property is read-only for
`NodeList` instances.

So, create a new array of the required size and copy the elements of the
`NodeList` in a loop.

The tests that fail on this branch also fail on `master`.