Fix Groonga::NoSuchColumn error when interating ResultSet

This PR fixes `Groonga::NoSuchColumn: no such column: <"_score">` error that is raised when iterating ResultSet that is created with ActiveGroonga::Base#select of Groonga::Hash table.

I am not sure if I am using '#select' correctly, but the following script raises such exception.

```ruby
require 'active_groonga'

db_path = '/tmp/db.groonga'
begin
  Groonga::Database.new(db_path)
rescue Groonga::NoSuchFileOrDirectory
  Groonga::Database.create(path: db_path)
end

Groonga::Schema.remove_table('sites') if Groonga['sites']
Groonga::Schema.create_table('sites', type: :hash) do |table|
  table.text('title')
end

class Site < ActiveGroonga::Base
  table_name('sites')
end

[
  ['http://www.yahoo.com', 'Yahoo!'],
  ['http://www.google.com', 'Google']
].each do |uri, title|
  Site.new(key: uri, title: title).save
end

Site.select.each { |r| p r }
```

[This  commit](dd879e089fb71ab0f61044f9175e8146df6a8a23) seems to be addressing the same issue but for Groonga::Array table.