"initial" Scene prop value is not taking effect

### Version
Tell us which versions you are using:

- react-native-router-flux v4.?.?
- react v16.?.?
- react-native v0.?.?

### Expected behaviour

### Actual behaviour

### Steps to reproduce
For non-obvious bugs, please fork this component, modify Example project to reproduce your issue and include link here.
1.
2.
3.

### Reproducible Demo

Please provide a minimized reproducible demonstration of the problem you're reporting.

Issues that come with minimal repro's are resolved much more quickly than issues where a maintainer has to reproduce themselves.