First Transition on Observable missing initial state

When using `onTransition` for an Observable, the first transition from the initial state to a new state is missed.  All subsequent transitions work correctly.

For example:

```
let value = ObservableInput(1.0)
value.onTransition { from, to in
  print("\(from) -> \(to)")
}
value.set(2.0) // prints: nil -> 2.0
value.set(3.0) // prints: 2.0 -> 3.0
```