fix(react-router): Allow string and object refs in withRouter (#6680)

* fix(react-router): Don't warn about ref objects

* fix(react-router): Don't warn about string refs

Author: eps1lon

packages/react-router/modules/withRouter.js

@@ -36,7 +36,11 @@ function withRouter(Component) {
 
   if (__DEV__) {
     C.propTypes = {
-      wrappedComponentRef: PropTypes.func
+      wrappedComponentRef: PropTypes.oneOfType([
+        PropTypes.string,
+        PropTypes.func,
+        PropTypes.object
+      ])
     };
   }