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 
A name avoiding the 'circle' word should be preferable to avoid all
possible confusion between a circle, which is a mathematical object and
a `circular` polygon, which is a polygon that is created looking like a
circle.

`ol.geom.Polygon.createCircularOnSphere` would be OK.