Ensuring StringIO's encoding in CSV.generate

[hanachin]

2.6

% ruby -EUTF-8:UTF-8 -ve 'require "csv"; p CSV.generate(String.new(encoding: Encoding::Big5), encoding: Encoding::Big5) {|csv| csv << ["漢字"] }.encoding'
ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-linux]
#<Encoding:Big5>

master

% ruby -EUTF-8:UTF-8 -ve 'require "csv"; p CSV.generate(String.new(encoding: Encoding::Big5), encoding: Encoding::Big5) {|csv| csv << ["漢字"] }.encoding'
ruby 2.7.0dev (2019-11-23T07:06:30Z master b563439274) [x86_64-linux]
#<Encoding:UTF-8>

This branch

% bundle exec ruby -EUTF-8:UTF-8 -ve 'require "csv"; p CSV.generate(String.new(encoding: Encoding::Big5), encoding: Encoding::Big5) {|csv| csv << ["漢字"] }.encoding'
ruby 2.7.0dev (2019-11-23T07:06:30Z master b563439274) [x86_64-linux]
#<Encoding:Big5>

[kou]

Thanks!