Insurance contracts no bar to recovery.

No contract of employment, insurance, relief benefit, or indemnity for injury or death entered into by or on behalf of any employee, nor the acceptance of any such insurance, relief benefit, or indemnity by the person entitled thereto, shall constitute any bar or defense to any action brought to recover damages for personal injuries to or death of such employee; provided, however, that upon the trial of such action against any common carrier the defendant may set off therein any sum it has contributed toward any such insurance, relief benefit, or indemnity that may have been paid to the injured employee, or, in case of his death, to his personal representative.