§ 6. Defense; insurance fund. An employer who shall have contributed to an insurance fund created and maintained for the mutual purpose of indemnifying an employee for personal injuries, for which compensation may be recovered under this article, or to any relief society or benefit fund created under the laws of this state, may prove in mitigation of damages recoverable by an employee under this article such proportion of the pecuniary benefit which has been received by such employee from such fund or society on account of such contribution of the employer, as the contribution of such employer to such fund or society bears to the whole contribution thereto.