Pushing nil asynchronously to short-circuit a stream cause deadlock

This code doesn't work. "done" is never printed.

``` javascript
_([1])
.consume(function (err, x, push, next) {
if (err) {
push(err);
next();
} else if (x === _.nil) {
push(null, _.nil);
} else {
setTimeout(function () {
push(null, _.nil);
}, 0);
}
})
.done(function () {
console.log('done');
});
```

Spotted when looking at #558.

Start by running the JavaScript reproduction in the issue and compare its behavior with the related discussion in #558. Trace how consume handles the asynchronous push of _.nil; done should be printed without the stream deadlocking.

Written by the indexing model from the issue text.