WASHINGTON (Reuters) - The U.S. Supreme Court on Monday sided with a company that amended a collective bargaining agreement to force retirees to pay toward healthcare costs, throwing out a lower-court ruling that favored the former employees who objected to the change. On a unanimous vote, the nine-member court handed a win to M&G Polymers USA, a subsidiary of Italy-based chemical company Mossi & Ghisolfi International, by sending the case back for further proceedings in the Cincinnati-based 6th U.S. Circuit Court of Appeals. Nearly 500 plaintiffs from Ohio who had worked at the M&G polyester plant in Apple Grove, West...