A few bug fixes.

Fixes #2 - holes must be detected by looking at the winding order, since
regrettably the order of rings in a polygon record is not significant.
Furthermore, this requires checking which clockwise (exterior) ring contains a
given counterclockwise (interior) hole using a point-in-polygon check, and thus
is sensitive to degenerate and coincident polygons.

Also fixes a bug where missing string properties were reported as the empty
string rather than null.