Writable computed out of sync

Not sure if this is a bug or intended behaviour, but the following occurs in latest versions of both knockout and tko:

```js
o = ko.observable()
c = ko.pureComputed({ read: o, write: o })
c.subscribe(() => o('c'))
c('o')
o() // 'c'
c() // 'o'
```

I would expect `c() === o()` always, since `c()` should always read the value of `o()`.