calling navigationBar's overridden ref function

Summary: Before that it was not possible to get a ref to a navigation bar (unless using Navigator's internal `_navBar` prop)
Closes https://github.com/facebook/react-native/pull/3755

Reviewed By: svcscm

Differential Revision: D2674315

Pulled By: nicklockwood

fb-gh-sync-id: 26120f7bcbb675e8217b8bd963dcc6ed314d4ba3

Libraries/CustomComponents/Navigator/Navigator.js

@@ -1087,7 +1087,10 @@ var Navigator = React.createClass({
       return null;
     }
     return React.cloneElement(this.props.navigationBar, {
-      ref: (navBar) => { this._navBar = navBar; },
+      ref: (navBar) => {
+        this.props.navigationBar.ref instanceof Function && this.props.navigationBar.ref(navBar);
+        this._navBar = navBar;
+      },
       navigator: this._navigationBarNavigator,
       navState: this.state,
     });