Fix a bug that drilldown value can't be got by _key on command_version=2

We always used code multi-keys drilldown for single key drilldown case
on command_version=2. The problem isn't occurred on command_version=1.

command_version=1 is the current stable version. command_version=2 is
the current development version.

GitHub: fix ranguba/rroonga#38

Reported by YUKI "Piro" Hiroshi. Thanks!!!