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 
> The reprojection of an ol.geom.Circle is a circle.

Depends on whether you are talking about a circle on a sphere or a circle geometry. But I don't think this discussion takes us any further. I think we have agreed already that this pull request is fine, and we can improve documentation later if we see that users run into issues.