Non-4326 GeoJSON

#880 gets projection from the parser, which for GeoJSON is set to 4326. However, GeoJSON can have a crs object http://geojson.org/geojson-spec.html#coordinate-reference-system-objects
I don't know how widely this is used, but if present it should be parsed and also written out again on write.
At the least, ISTM the docs should specify that only 4326 GeoJSON is currently handled.