Added a way to change the objects are serialized CustomJsonSerializerSettings

[jjchiw]

I had a problem with the way the the Date was serialized and saved in the database. I needed to be saved with the format ISO 8601....

So I added a function in DatabaseSharedSetting to change the JsonSerializerSettings

var setting = new DatabaseSharedSetting();
...
setting.CustomJsonSerializer = (db, converters) =>
{
    return JsonSerializer.Create(
        new JsonSerializerSettings
        {
            ContractResolver =  ArangoDB.Client.Serialization.DocumentContractResolver.GetContractResolver(db),
            Converters = converters,
            DateParseHandling = DateParseHandling.None,
            DateTimeZoneHandling = DateTimeZoneHandling.Utc
        });
};

[ra0o0f]

thanks again @jjchiw for making the driver better. i will look at these changes asap. but just one thing to say is that ArangoDB is using json binary serializer called VelocyPack and its being used in their official java client. Json.NET support will remain in the c# client but i will add support for VelocyPack in the future. so these settings may change in the future to work with both of them