fix className is null

Author: jljsj33

package.json

@@ -54,7 +54,7 @@
   "dependencies": {
     "object-assign": "4.0.x",
     "raf": "3.1.x",
-    "rc-tween-one": "~0.6.19",
+    "rc-tween-one": "~0.8.0",
     "tween-functions": "1.0.x"
   }
 }

src/ScrollLink.jsx

@@ -174,6 +174,7 @@ ScrollLink.propTypes = {
 ScrollLink.defaultProps = {
   component: 'div',
   duration: 450,
+  className: '',
   active: 'active',
   showHeightActive: '50%',
   ease: 'easeInOutQuad',