“ In this case appellants contend that the San Francisco Milk Ordinance violates the Fourteenth Amendment because it requires non-pasteurized raw milk sold in San Francisco to be certified by, and to conform to standards prescribed by, the Milk Commission of the San Francisco Medical Society, instead of by a public board or officer, while at the same time prohibiting the sale of all other non-pasteurized milk, including 'guaranteed raw milk' which appellants allege is the same as certified raw milk. ”