Fix non-numeric transitionable

@marklu 

Transitioning non-numeric things like `size : [200, undefined]` currently gives incorrect behavior. I wonder if this should be fixed in `Transitionable`, rather than `TweenTransition` (PhysicsTransitions would also need to be fixed).

The approach I currently see is to strip out the non-numeric values on `.set` and add them back in on `.get`. This does a lot of array management however. Any other ideas?

Note: this will be a good JIT improvement, the current approach already improves the `_interpolate` performance