Unnecessary warning: An <IndexRoute> is not allowed for <TabsRoute>?

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### Version

| Dependency | Version |
| ------------------- | ------------------- |
| react-router-native | 2.0.0-beta.0 |
| react-router | 3.0.0 |
| react-native | 0.37.0 |

### Expected Behavior

First of all, thank you for creating this library and sharing with the community.

We are migrating from the `react-router-native-flux` library. I have an IndexRoute in a TabsRoute that sets the initial active route, similar to an `IndexRedirect` from react-router.

```
const to = (value) => (nextState, replace) => replace(value);

...

```

The `IndexRoute` definition is working just fine, except I am seeing a warning:

```
Warning: An is not allowed for
```

### Actual Behavior

I believe an IndexRoute should continue to work in this configuration, but with no warning.

### Steps to Reproduce

See router configuration above.