Parent title is changed to child title when back is pressed

### Version
Tell us which versions you are using:
- react-native-router-flux v4.0.6
- react v16.8.3
- react-native v0.59.9

### Expected behaviour
When on the child screen of a listview, pressing the back button doesn't change the title of the parent screen

### Actual behaviour
The title of the parent screen is changed to the child screen's title when the back button is pressed

I'm not sure this is a bug, it might be that I don't know how to make it function like I want.