code: @Override
      protected String convertToString(Object value) throws Throwable {
  		GeoPt geoPt = (GeoPt) value;
  		return new BigDecimal(geoPt.getLatitude()) + "," + new BigDecimal(geoPt.getLongitude());
      }
docs: Returns {@code com.google.appengine.api.datastore.GeoPt} value as string
  representation.