[gh-21] Potential fix for map-stream integration with multipipe.

[ashmind]

This is a potential fix for #21.

All existing test pass, but unfortunately I don't know streams well to do a proper new test.

I've inlined wrappedMapper — to reduce confusion between the callback it took (which was always next) and a new callback that write takes.

[dominictarr]

I think there should be a simpler solution that doesn't require changes to map-stream, I suspect that emitting a finish event should be enough

[ashmind]

@dominictarr I'm not really a stream expert — the callback approach fixed the problem and made sense for me — but if you know a better fix all the better.