sort: fix a bug that sort by _id may be wrong
sort: fix a bug that sort by _id may be wrong

kenhys/groonga,kenhys/groonga,naoa/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,naoa/groonga,groonga/groonga,groonga/groonga,naoa/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,naoa/groonga,naoa/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga