Improve handling a datetime field in a sub-type

[steco]

I ran into an issue where a subtype containing a date field was not deserialised correctly (it appeared that the timezone of the date was changed to my local machine's timezone). This is an attempt at a fix based on a workaround I found for this issue, but would appreciate it if there was a better solution.

[manuc66]

Hello,

The issue should apparently not have been solved in this library, if you want the deserialization to happen as you expect you should deserialize with that settings;

            var obj = JsonConvert.DeserializeObject<MainClass>(json, new JsonSerializerSettings
            {
                DateParseHandling = DateParseHandling.DateTimeOffset
            });

The way it has been solved cause #120