Whenever a contractor, laborer, design professional, or materials supplier has expended labor, services, or materials under contract for the improvement of real property, and where due and just demand has been made by certified or registered mail for payment for the labor, services, or materials under the terms of any regulation, undertaking, or statute, it is the duty of the person upon whom the claim is made to make a reasonable and fair investigation of the merits of the claim and to pay it, or whatever portion of it is determined as valid, within forty-five days from the date of mailing the demand. If the person fails to make a fair investigation or otherwise unreasonably refuses to pay the claim or proper portion, he is liable for reasonable attorney’s fees and interest at the judgment rate from the date of the demand.