In any action to recover damages for negligently causing the death of a person, or for negligently causing personal injury or property damage, it shall be presumed that such person whose death was caused or who was injured or who suffered property damage was, at the time of the commission of the alleged negligent act or acts, in the exercise of reasonable care. If contributory negligence is relied upon as a defense, it shall be affirmatively pleaded by the defendant or defendants, and the burden of proving such contributory negligence shall rest upon the defendant or defendants.