Fix NestedRecord, by improving on Tyler work

As discussed on IRC this week I had been working on NestedRecords at about the same time Tyler pushed a fix for a failing test. I wrote a [comment](https://github.com/sproutcore/sproutcore/commit/9efed3b7b4fee3ad8f2fdee08d91ed7d9a04f220#commitcomment-4042931) on his commit pointing out that there were other cases that would have not worked and provided tests to prove it.

This pull request should fix most if not all use cases for NestedRecords. If the pull request is accepted it is my intention to add more unit test coveing other cases, so please before merging wait for the other unit tests.