Two variables with their `.toString()` equal may not be the same value.

    function proxy(fn) {
        return function () {
            fn();
        }
    };
    var a = proxy(function () {
        alert('a');
    });
    var b = proxy(function () {
        alert('b');
    });
    var $body = $(document.body);
    $body.on('click', a).on('click', b);
    // Both listener will work.
    $body.off('click', a);
    // Both listener will be mistakenly removed.