effects core: the second argument passed to effect methods wasn't checked for being a callback function (fixes #4181 - Effects methods don't allow callback as the second parameter)

Author: Paul Bakaus

ui/effects.core.js

@@ -136,7 +136,7 @@ function _normalizeArguments(a, m) {
 	var o = a[1] && a[1].constructor == Object ? a[1] : {}; if(m) o.mode = m;
 	var speed = a[1] && a[1].constructor != Object ? a[1] : (o.duration ? o.duration : a[2]); //either comes from options.duration or the secon/third argument
 		speed = $.fx.off ? 0 : typeof speed === "number" ? speed : $.fx.speeds[speed] || $.fx.speeds._default;
-	var callback = o.callback || ( $.isFunction(a[2]) && a[2] ) || ( $.isFunction(a[3]) && a[3] );
+	var callback = o.callback || ( $.isFunction(a[1]) && a[1] ) || ( $.isFunction(a[2]) && a[2] ) || ( $.isFunction(a[3]) && a[3] );
 
 	return [a[0], o, speed, callback];