Undefined second parameters triggers getter instead of setter

[MrMamen]

Consider the following function:

function f(bar) {    
    var $div = $("<div />").data("foo", bar);
}

The $div variable should be the DOM object, since data with two parameters is a setter and returns itself. However if the function f is called without a parameter, "bar" is undefined, and data triggers get method, and returns undefined instead.

See jsFiddle here:  http://jsfiddle.net/mrmamen/3Xqk7/

This applies not only to data, but also to attr, css, and probably most if not all of jQuerys methods with have setter and getter.

Note: You can detect the difference between these: test(foo) and test(foo, undefined) be checking arguments.length.

[timmywil]

Duplicate of #5571.