Convert className in join

We don't need to call toString() on parent component when passing className down.

Author: Joozty

src/utils/class-names.js

@@ -27,7 +27,8 @@ class ClassNames {
   }
 
   join(className) {
-    this.appendix = className
+    if (className instanceof ClassNames) this.appendix = className.toString()
+    else this.appendix = className
     return this
   }