Redirected streams do not send nil

See this example:

```
var s1 = highland([1]);
var s2 = highland([2]);

var out = s1.concat(s2);

out.toArray(function(arr) { console.log(arr); }); // [ 1, 2 ]
console.log(s1.ended); // true
console.log(s2.ended); // true
console.log(out.ended); // undefined - expected true?
```

I was expecting that `out.ended` would also be true, since both its sources have ended?