Revised newsletter spec.rb

I have do some modification following your suggestions. I check the initialized variables, and the updates values after calling the method. Because I couldn't call the function explicitly, I just try to set the mode to instant, and do_send to be true, which might send newsletters for me automatically. However, I can only check the last_user_id is exactly the last person's id. I set the CHUNK_SIZE 100, but I cannot prove any difference, if there are more people than 100. I really appreciate your review and help.