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 
Do we really need the function implementation here? [Elsewhere](https://github.com/openlayers/ol3/blob/master/src/ol/format/geojsonformat.js#L345-L355) we just provide the "overriden" docs. Can we use the same here?