Line.to_geo fix + specs, Polygon.to_geo specs

please check if it's ok for you, especially this stuff (abstracted in GeometryField 'cause it's repeated in both `Line.to_geo` and `Polygon.to_geo`):

https://github.com/nofxx/mongoid_geospatial/pull/26/files#L0R29
```rb
    class GeometryField
      private
      def points
        self.map do |pair|
          RGeo::Geographic.spherical_factory.point(*pair)
        end
      end
    end
```