fix string type system variable implementation.
fix string type system variable implementation.

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