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 
You are confusing Circle and Polygon.

A 'circular' polygon is not a circle.

The reprojection of an ol.geom.Polygon, be it circular or not, is a polygon.
The reprojection of an ol.geom.Circle is a circle.