map-stream never ends when used inside of multipipe

[ashmind]

I'm using a Gulp plugin called gulp-svg2png which uses map-stream internally.

When I use it with the multipipe, the stream end never happens (though gulp does exit). I have narrowed it down to map-stream not using the third callback argument of stream.write: https://github.com/dominictarr/map-stream/blob/master/index.js#L89. I'm not a Node stream expert, but it seems that if I accept the callback and call it when required, then everything works as expected.

[ashmind]

Removed potential solution from the description above -- I have a better solution in a fork so I'll PR it.