Improve preventDefault fix for rails-ujs

Improves 049a337:

* Attempt native `preventDefault()` before stepping in
* Fix that calling `preventDefault()` more than once would throw an error
* Fix that non-cancelable events could be canceled

actionview/app/assets/javascripts/rails-ujs/utils/event.coffee

@@ -7,17 +7,22 @@
 CustomEvent = window.CustomEvent
 
 if typeof CustomEvent isnt 'function'
-  CustomEvent = (event, params) ->
+  window.CustomEvent = (event, params) ->
     evt = document.createEvent('CustomEvent')
     evt.initCustomEvent(event, params.bubbles, params.cancelable, params.detail)
-    # IE does not set `defaultPrevented` when `preventDefault()` is called on CustomEvents
-    # http://stackoverflow.com/questions/23349191/event-preventdefault-is-not-working-in-ie-11-for-custom-events
-    evt.preventDefault = ->
-      Object.defineProperty this, 'defaultPrevented', get: ->
-        true
     evt
+
   CustomEvent.prototype = window.Event.prototype
 
+  # Fix setting `defaultPrevented` when `preventDefault()` is called
+  # http://stackoverflow.com/questions/23349191/event-preventdefault-is-not-working-in-ie-11-for-custom-events
+  { preventDefault } = CustomEvent.prototype
+  CustomEvent.prototype.preventDefault = ->
+    result = preventDefault.call(this)
+    if @cancelable and not @defaultPrevented
+      Object.defineProperty(this, 'defaultPrevented', get: -> true)
+    result
+
 # Triggers a custom event on an element and returns false if the event result is false
 # obj::
 #   a native DOM element