Don't install unsupported event methods on the delegate proxy object

If `event.stopImmediatePropagation()` is not supported by the current
browser, don't install that method either on the proxy object used
during event delegation. This helps getting more descriptive error
messages.

Closes madrobby#774

src/event.js

@@ -121,6 +121,7 @@
       if (!ignoreProperties.test(key) && event[key] !== undefined) proxy[key] = event[key]
 
     $.each(eventMethods, function(name, predicate) {
+      if (!event[name]) return
       proxy[name] = function(){
         this[predicate] = returnTrue
         return event[name].apply(event, arguments)

test/event.html

@@ -278,6 +278,31 @@ <h1>Zepto event tests</h1>
 
         this.el.trigger('myevent')
         t.assertIdentical(data, gotData)
+      },
+
+      testDelegateEventMethods: function(t){
+        var methodName,
+            eventMethods = {
+              preventDefault: 'isDefaultPrevented',
+              stopImmediatePropagation: 'isImmediatePropagationStopped',
+              stopPropagation: 'isPropagationStopped'
+            },
+            eventMethodResults = {}
+
+        $(document).on('click', 'div', function(event){
+          $.each(eventMethods, function(methodName, predicate){
+            if (event[methodName]) {
+              event[methodName]()
+              eventMethodResults[predicate] = event[predicate]()
+            }
+          })
+        })
+
+        click($('<span>').appendTo(this.el))
+
+        $.each(eventMethodResults, function(predicate, value){
+          t.assertTrue(value, predicate)
+        })
       }
     })
   })()