Fix issue with StackRouter popToTop (#3451)

Previously the state was getting squashed, in this case it would destroy the routeName of the state, which was a route for the parent navigator, who could no longer render properly.

examples/NavigationPlayground/js/SimpleStack.js

@@ -27,6 +27,7 @@ class MyNavScreen extends React.Component<MyNavScreenProps> {
           onPress={() => navigation.navigate('Profile', { name: 'Jane' })}
           title="Go to a profile screen"
         />
+        <Button onPress={() => navigation.popToTop()} title="Pop to top" />
         <Button
           onPress={() => navigation.navigate('Photos', { name: 'Jane' })}
           title="Go to a photos screen"