ARROW-7474: [Ruby] Improve CSV save performance
ARROW-7474: [Ruby] Improve CSV save performance

Closes #6106 from kou/ruby-csv-save-improve and squashes the following commits:

95c1e99dc <Sutou Kouhei> ARROW-7474:  Improve CSV save performance

Authored-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
Signed-off-by: Yosuke Shiro <566f3c030d8064e2a2a198dafd4d52e9d358b1f1@gmail.com>

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