Consistent circle transform.

Depending on compilation flags, applying a transform used to either:
- work;
- fail throwing an abstract method not implemented message;
- fail silently.

Now it should consistently work, like the other geometries.

Fix #2680 
Thinking about it I don't think `ol.geom.Polygon.circular` is relevant to the discussion. That function is about creating an approximation of a circle on the surface of a sphere. So it is not really related to `ol.geom.Circle`. We could have an `ol.geom.Polygon.createCircle` factory function, which would work on the cartesian plane. But `ol.geom.Polygon.circular` is something different. So I'd actually not mention it here. @ahocevar, does it make sense to you?