Fix defaultPrevented tests for Android 2.3

This property is not present in Android 2.3 so it shouldn't be used in
tests. Instead, use `isDefaultPrevented()` which is guaranteed to be
cross-platform.

test/zepto.html

@@ -2497,7 +2497,7 @@ <h1>Zepto DOM unit tests</h1>
             return false
           })
           .bind('click', function (e) {
-            prevented = e.defaultPrevented
+            prevented = e.isDefaultPrevented()
           })
           .trigger('click')
 
@@ -2560,21 +2560,18 @@ <h1>Zepto DOM unit tests</h1>
       testTriggerEventCancelled: function(t){
         var event = $.Event('custom'),
           element = $('<div/>'),
-          defaultPrevented = false,
           isDefaultPrevented = false
 
         t.refute(event.isDefaultPrevented())
 
         element.bind('custom', function(e){
           e.preventDefault()
-          defaultPrevented = e.defaultPrevented
           isDefaultPrevented = e.isDefaultPrevented()
         })
 
         element.trigger(event)
 
         t.assertTrue(event.isDefaultPrevented())
-        t.assertTrue(defaultPrevented)
         t.assertTrue(isDefaultPrevented)
       },