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 
I find the last sentence unclear. How about that:

```
Note that transforming a circle from one coordinate reference system to another doesn't necessary yield a circle. For example transforming a circle from EPSG:4326 to EPSG:3857 does not yield a circle.

Internally a circle is represented by two points: the center of the circle `[cx, cy]`, and the point to the left of the circle `[cx + r, cy]`. This `transform` function just transforms these two points. So the resulting geometry is also a circle, and that circle doesn't correspond to the shape that would be obtained by transforming every point of the original circle.

See also {@link ol.geom.polygon.circular}.
```