`filter_features` output for MultiPolygons/MultiLineStrings/MultiPoints is not valid GeoJSON

[e-n-f]

filter_features decomposes MultiPolygons, MultiLineStrings, and MultiPoints into sub-features, but its output is not valid GeoJSON because these sub-features do not have a properties.

[dnomadb]

@ericfischer in hindsight, I think the best way to handle this would have been to always deal with the geometry object only, as this is only what is needed internally for supermercado. But, I don't want to break any functionality so I added test + a PR to simply pass through whatever is provided to said function (as it currently works for non-Multi* geometry types): #46