Fix Polygon and LineString serialization

The first commit fixes an invalid element name in `GeoJSONPolygonFeatureGeometryFromPolygon()` – the coordinates are now labeled "coordinates" instead of "geometry".

Also, I changed `GeoJSONPolygonFeatureGeometryFromPolygon()` and `GeoJSONLineStringFeatureGeometryFromPolyline()` to write out geographic coordinates instead of `MKMapPoint` values.

Please have a look, thanks