Passing headerStyle doesn't actually style the header

[kennetpostigo]

When I pass a style to the headerStyle prop on StackNavigator.Screen it doesn't actually style anything.

[broerjuang]

I try to made a second PR to fix this, hope can contribute more 😸

[knowbody]

@broerjuang great PR, thank you for contributing!
@kennetpostigo could you try master now and see if the issue is gone? It works for me but I'll wait for you to report back, as you opened the issue. If all is good we will close the issue.

[kennetpostigo]

I won't have time to try it out until this weekend, but I think if it works for you it should be enough to close this issue.