Roundtrip foreign members

[1ec5]

A GeoJSONObject should roundtrip from and to JSON in its entirety, including any foreign members (JSON properties outside of the properties object).

When decoding from JSON, any property in KeyedDecodingContainer.allKeys but not CodingKeys should be stored in a userInfo property of type JSONObject, which would get encoded back to JSON as the original properties. If the userInfo dictionary has a key that’s reserved in CodingKeys, such as property or id, the Encodable implementation could ignore it.

[1ec5]

Per mapbox/mapbox-directions-swift#637 (comment), the CodingKey implementation would be an extensible enumeration so that allKeys can return everything, not just what we’ve statically defined for the type.