md5("code=a94a110d86d2452eb3e2af4cfb8a3828shop=some-shop.myshopify.comtimestamp=1337178173" + shared_secret) == "31b9fcfbd98a3650b8523bcc92f8c5d2"
must be
md5(shared_secret+"code=a94a110d86d2452eb3e2af4cfb8a3828shop=some-shop.myshopify.comtimestamp=1337178173" ) == "31b9fcfbd98a3650b8523bcc92f8c5d2"