Strange behaviour of a forked stream

Check the code below:
```
const H = require('highland')

const IntervalStream = function(time, value) {
return H(function(push, next) {
push(null, value)
setTimeout(
next,
time
)
})
}

const stream = IntervalStream(100, 1).scan1(H.add)
//.tap(x => x)// HACK: doesn't work without tap <-----------------------------

stream.fork().filter(i => i % 5).map(() => '.').pipe(process.stdout)
stream.fork().reject(i => i % 5).map(() => '!').pipe(process.stderr)
```
If run it:
```
>node generator.js
................^C
```
but if I uncoment the line with .tap I see:

```
>node generator.js
....!....!...
```

I believe it's a kind of a bug.
```
"dependencies": {
"highland": "^2.10.5"
}
```

Start by running generator.js with Highland 2.10.5, comparing the forked stream with and without the tap call. Trace how scan1, fork, filter, and reject consume values, then check existing stream tests for related fork behavior. Done means the differing outputs are explained and the intended fork behavior is covered by a regression test.

Written by the indexing model from the issue text.