[jQuery] Problem with extend() when source object contains functions

[GazQuest]

Run this code :

var empty = {};
var defaults = { validate: false, limit: 5, name: "foo", buttons :
{}};
var options = { validate: true, name: "bar"};
var settings = $.extend(empty, defaults, options);

settings.buttons['Ok']  =
    function()
    {
        alert( "TEST");
    };

After running this settings.buttons contains ["Ok"] as expected BUT so
does defaults.buttons.

I only set settings.buttons["Ok']. Somehow extend() causes this
behaviour can anyone explain why?