An Illinois consumer got the go-ahead Wednesday to pursue a proposed class-action lawsuit accusing a manufacturer of grossly understating the amount of folic acid contained in a dietary supplement.In a written opinion, U.S. District Judge John J. Tharp Jr. declined to dismiss the proposed class-action suit that David Chavez filed against Church & Dwight Co. Inc.Tharp rejected the argument that federal labeling law pre-empts the claims Chavez brought under various state laws.But Tharp dismissed claims brought on behalf …