bitovi/canjs — pull request #523 by justinbmeyer (2013-11-01 18:35:05+00:00)
Title: supports cycles in maps and fixes #521

also supports:

```js
var me = {name: "Justin"}
var references = {husband: me, friend: me}
var ref = new can.Map(references)

ok( ref.attr('husband') === ref.attr('friend'), "multiple properties point to the same thing")
```