Component not unmounting on goBack()

Hi, I am facing an issue with a fairly simple usage of this module. I am finding that after calling goBack() on the router it does not trigger a componentWillUnmount(). Here is my route configuration:

```

}/>
}/>
}/>

```

What seems to happen is, if I use router.push() to go from path "/" to "/chat/:name/:id" I get a call to componentDidMount() and componentWillReceiveProps(). On calling router.goBack(), componentWillUnmount() is not triggered. If I go back to the exact same chat link then it will call componentDidMount() and componentWillReceiveProps() again.

### Version

| Dependency | Version |
| ------------------- | ------------------- |
| react-router-native | 2.0.0-beta.0 |
| react-router | N/A |
| react-native | 0.37.0 |