Convert ordered factors to character, add SpatialPointsDataFrame method

writeOGR fails with ordered factors, so convert data to character before calling (https://github.com/ateucher/togeojson/commit/99ae95e3064540b81cc17d6988b4dbfc8b5e1d60)

Also adds method for SpatialPointsDataFrame (https://github.com/ateucher/togeojson/commit/258c2995a7bdc4c540e9ae9d064bfef9fb394ff2)