Accept function as data for on event handler

Move the parameter data to callback, if callback is undefined or data is
false. As the data can hold any type, including function, the check is
done on the callback, to see if it is undefined, before moving the
parameters.

Fixes madrobby#1042

src/event.js

@@ -182,7 +182,7 @@
 
     if (!isString(selector) && !isFunction(callback) && callback !== false)
       callback = data, data = selector, selector = undefined
-    if (isFunction(data) || data === false)
+    if (callback === undefined || data === false)
       callback = data, data = undefined
 
     if (callback === false) callback = returnFalse

test/event.html

@@ -274,6 +274,21 @@ <h1>Zepto Event unit tests</h1>
         t.assertEqual(1, numArgs)
         t.assertIdentical(data, gotData)
       },
+
+      testHandlerWithFunctionAsData: function(t){
+        var data = function () { console.log('foo') },
+            gotData, numArgs
+
+        $(document).on('click', data, function(event){
+          gotData = event.data
+          numArgs = arguments.length
+        })
+        t.assertUndefined(gotData)
+
+        click(this.el)
+        t.assertEqual(1, numArgs)
+        t.assertIdentical(data, gotData)
+      },
 
       testDelegatedWithData: function(t){
         var data = {}, gotData, numArgs