Prevent observer from observing its own deallocation

I ran into the problem from #631, where I had bound a command's `canExecute` status to a button's `enabled` property, like so:

```objc
    RAC(self.doneButton.enabled) = RACObserve(self.processSignUpCommand.canExecute);
```

When the host controller was deallocated, the KVO wrapper recognized that the first component of the key path, `self`, was deallocated, and proceeded to attempt to assign `nil` to the button's `enabled` property.

This PR prevents the observer from observing on its own deallocation. An observer about to be deallocated shouldn't cause itself to receive a KVO notification triggered by its own deallocation.