PositionConverter no longer throws a NotImplementedException if passed an IPosition instead of Position

Author: BarryPSmith

src/GeoJSON.Net/Converters/PositionConverter.cs

@@ -57,7 +57,7 @@ public override object ReadJson(JsonReader reader, Type objectType, object exist
         /// <param name="serializer">The calling serializer.</param>
         public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
         {
-            if (value is Position coordinates)
+            if (value is IPosition coordinates)
             {
                 writer.WriteStartArray();