Accept children of all types for NavigationHeaderTitle

Summary:
`<Text>` accepts more than just string as its children; and it's handy to be able to style nav titles without a proptype warning throwing
Closes https://github.com/facebook/react-native/pull/8338

Differential Revision: D3719045

fbshipit-source-id: a02d181b5e133dbe467fbaca794f0d2119be2da7

Libraries/CustomComponents/NavigationExperimental/NavigationHeaderTitle.js

@@ -73,7 +73,7 @@ const styles = StyleSheet.create({
 });
 
 NavigationHeaderTitle.propTypes = {
-  children: React.PropTypes.string.isRequired,
+  children: React.PropTypes.node.isRequired,
   style: View.propTypes.style,
   textStyle: Text.propTypes.style
 };