madrobby/zepto — pull request #458 by i8ramin (2012-03-28 22:01:15+00:00)
Title: extend should not copy over undefined values

jQuery does not copy over undefined values when extending objects. To maintain compatibility, I've made Zepto behave the same.

http://james.padolsey.com/jquery/#v=1.6.2&fn=jQuery.extend

Scroll down towards the bottom where it says "Don't bring in undefined values"