Added test for intermediate nil values triggering ignoring an update that never happens.
Added NSString(RACKeyPathUtilities).
Fix update being applied to key paths when there are nil intermediate values.
Merge branch 'master' into racobservablepropertysubject-fix-keypath
Add rac prefix to category methods and nicer comments.
Merge pull request #470 from ReactiveCocoa/racobservablepropertysubject-fix-keypath

RACObservablePropertySubject tries to update key path with nil intermediate values and fails.