UpdateBuilder.Combine cannot be used with UpdateBuilder<T> and vice versa

It is not possible to pass an UpdateBuilder<T> into the UpdateBuilder.Combine method, or an UpdateBuilder into an UpdateBuilder<T>.Combine method. It is Perhaps the Combine methods should take IMongoUpdate as the argument type instead, or UpdateBuilder<T> could be made to inherit from UpdateBuilder and the UpdateBuilder<T>.Combine method switched to take UpdateBuilder instead of UpdateBuilder<T>.