Html.disabled can be undefined

Calling `.disabled` on elements like `HTMLImageElement`, `HTMLDivElement`, `HTMLParagraphElement` type-checks and causes this nastiness (at least in Chrome): ``` Uncaught scala.scalajs.runtime.UndefinedBehaviorError: An undefined behavior was detected: undefined is not an instance of java.lang.Boolean ```

Author: japgolly

src/main/scala/org/scalajs/dom/raw/Html.scala

@@ -3646,7 +3646,7 @@ abstract class HTMLElement extends Element {
  var onloadeddata: js.Function1[Event, _] = js.native
  var onbeforedeactivate: js.Function1[UIEvent, _] = js.native
 
- var disabled: Boolean = js.native
+ var disabled: js.UndefOr[Boolean] = js.native
  var onactivate: js.Function1[UIEvent, _] = js.native
  var onselectstart: js.Function1[Event, _] = js.native
  var ontimeupdate: js.Function1[Event, _] = js.native