don't throw if readStream.destroy called more than once

[mmckegg]

It's too easy to accidentally call destroy on a readStream, and it's unforgiving. Had some greif when using dominictarr's map-reduce which assumes that it's safe to call.

[juliangruber]

👍

[rvagg]

related #82

[rvagg]

btw I think the problem is a bit bigger than this because of the async end() call to the iterator. Just fiddling with alternatives to the this._status dogs-breakfast.

[rvagg]

@mmckegg thanks so much for investigating this and providing a fix. Your code is fine but I'm going to close this in favour of #84 which is a little more complete (tho you're more than welcome to critique it!). I'll push out a new release in a moment that should sort this out; it'll be 0.6.1.