Fix sequence for the stream of stream of arrays case.

Sequence does a two-level flatten if you have a Stream of Stream of Arrays. Basically, this test fails:

    _([
        _([1,2]),
        _([3]),
        _([[4],5])
    ]).sequence().toArray(function (xs) {
        test.same(xs, [1,2,3,[4],5]);
        test.done();
    });

It emits `[1,2,3,4,5]` instead.