§ 78. Mitigating circumstances in action for libel or slander. In an action for libel or slander the defendant may prove mitigating circumstances, including the sources of his information and the grounds for his belief, whether or not he has pleaded or attempted to prove any defense. A defendant in default for want of an answer may prove such mitigating circumstances upon a reference or inquiry to ascertain the amount of the plaintiff's damages. Matter tending only to mitigate or reduce damages is a partial defense and may be set forth in the answer.