Disable spacebar toggle when disabled

Fixes googlearchive/paper-checkbox#17

Author: dfreedm

paper-radio-button.html

@@ -133,6 +133,9 @@
     },
 
     tap: function() {
+      if (this.disabled) {
+        return;
+      }
       var old = this.checked;
       this.toggle();
       if (this.checked !== old) {