mysql57: support multiple column key and exact match case
mysql57: support multiple column key and exact match case

mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga