Default serialization options cannot be modified

[DiscoPYF]

The readme says that the default serialization options can be modified like so:

var serializer = new JsonNetApiClientSerialization();
serializer.DefaultOptions.IgnoreNullValues = false;

This doesn't work at the moment because ApiClientSerialization.DefaultOptions always returns a new instance of ApiClientSerializationOptions:

public virtual ApiClientSerializationOptions DefaultOptions => 
            new ApiClientSerializationOptions(false, true, false);

We should use assign the value instead of returning it:

public virtual ApiClientSerializationOptions DefaultOptions { get; } = 
            new ApiClientSerializationOptions(false, true, false);