fix Groonga::NoSuchColumn error when interating

Groonga::NoSuchColumn: no such column: <"_score"> is raised
when iterating ResultSet that is created with
ActiveGroonga::Base#select of Groonga::Hash table.
Merge pull request #15 from eitoball/fix_no_such_column__score_error

Fix Groonga::NoSuchColumn error when interating ResultSet

Patch by Eito Katagiri. Thanks!!!