Resolved issue with removeObserver and @each

[omghax]

Here's a patch for an issue I was running into when adding and removing content observers at runtime. I would start out by adding an observer for something like @each.name and then later on replace it with one on @each.age.

The problem is that SC._ChainObserver.destroyChain doesn't remove content observers, so when I removed the @each.name observer, "name" would still be in the _kvo_content_observed_keys set, and when I then added the @each.age observer, _setupContentObservers would loop through those keys and find "name", even though the observer itself had been set to null. It would then call _resumeChainObservingForItemWithChainObserver and pass in null as the second argument (observer), which would throw an error because it would try to access observer.property.

To fix this I added a special case for @each in destroyChain, which calls _removeContentObserver on the object. Tests are included.

[ColinCampbell]

Great stuff, thanks :)