Any private employer not otherwise included by section 115 and 118 may assume the liability for compensation and benefits imposed by this act upon employers. The purchase and acceptance by an employer of a valid compensation insurance policy, except in the case of domestics and agricultural employees, constitutes an assumption by him of such liability without any further act on his part, which assumption of liability shall take effect from the effective date of the policy and continue only as long as the policy remains in force, in which case the employer shall be subject to no liability other than workmen’s compensation as provided for in this act. Agricultural and domestic employees may be voluntarily included by specific indorsement to a workmen’s compensation policy in those cases where such coverage is not required.