add cascading attributes (#141)

* add cascading attributes

* fix wording of attr cascade test case

Author: mwcz

elements/rh-card/rh-card.js

@@ -36,7 +36,7 @@ class RhCard extends RHElement {
   }
 
   constructor() {
-    super(RhCard.tag);
+    super(RhCard);
   }
 }