Union serialization does not take into account predefined casing preferences

[Alejandro-SB]

When serializing, even if camel case serialization is preferred, UnionSerializer uses Pascal case to serialize property names.

Example:

public record UserNotFound(int UserId);
public record UsernameInUse(string Username);

[Union<UserNotFound, UsernameInUse>]
public partial class UserCreationError;

Would serialize to:
Current serialization:

{
    "UserNotFound": {
        "UserId": 1
    }
}

[Alejandro-SB]

Should be closed if #18 is finally done

[Alejandro-SB]

Closed with #20
Now it uses serialization as per #18