select: fix a bug that output_pretty isn't used for cache key
select: fix a bug that output_pretty isn't used for cache key

GitHub: #490

Reported by KITAITI Makoto. Thanks!!!

komainu8/groonga,komainu8/groonga,kenhys/groonga,cosmo0920/groonga,cosmo0920/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,cosmo0920/groonga,kenhys/groonga,komainu8/groonga,komainu8/groonga,komainu8/groonga,naoa/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,groonga/groonga,naoa/groonga,naoa/groonga,naoa/groonga,groonga/groonga,naoa/groonga,naoa/groonga,cosmo0920/groonga,cosmo0920/groonga,cosmo0920/groonga,groonga/groonga,groonga/groonga,cosmo0920/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,kenhys/groonga,cosmo0920/groonga