During the "normal" callback for new and old values, the ofObject:
parameter passed to the observer is the original receiver of the
addObserver:forKeyPath:options:context: call, but that isn't the case
with the call caused by the CPKeyValueObservingOptionInitial option.
This commit uses _targetObject, the receiver of the addObserver:...
call, rather than self, the _CPKVOProxy.
