Ch15 ex1: fix Process.|> implementation.

I think there are two bugs in the implementation, here's my test case:

```scala
def appendMinus1: Process[Int, Int] = Await(
  i => Emit(Seq(i), appendMinus1),
  Emit(Seq(-1), Halt()))

val p = appendMinus1 |> appendMinus1

assert (p(Stream(1,2,3)).toList == List(1,2,3,-1,-1))
```