pg gem: invalid byte sequence in UTF-8

test.rb

require 'pg'

# $ docker run --rm -p 5432:5432 -e POSTGRES_PASSWORD=password -d postgres:9.6

conn = PG.connect(host: 'localhost', port: 5432, user: 'postgres', password: 'password', dbname: 'postgres')

conn.exec <<SQL
drop table if exists test_copy_to;
create table test_copy_to (str varchar);
insert into test_copy_to (str) values ('Ä');
SQL

dec = PG::TextDecoder::CopyRow.new
conn.copy_data('COPY (select * from test_copy_to) TO STDOUT', dec) do
  while row = conn.get_copy_data
    puts ['(1)', row[0].inspect, row[0][0].inspect, row[0].encoding].join("\t")
  end
end

dec = PG::TextDecoder::CopyRow.new(type_map: PG::TypeMapByColumn.new([PG::TextDecoder::String.new]))
conn.copy_data('COPY (select * from test_copy_to) TO STDOUT', dec) do
  while row = conn.get_copy_data
    puts ['(2)', row[0].inspect, row[0][0].inspect, row[0].encoding].join("\t")
  end
end

dec = PG::TextDecoder::CopyRow.new(type_map: PG::TypeMapByColumn.new([PG::TextDecoder::Bytea.new]))
conn.copy_data('COPY (select * from test_copy_to) TO STDOUT', dec) do
  while row = conn.get_copy_data
    puts ['(3)', row[0].inspect, row[0][0].inspect, row[0].encoding].join("\t")

    str = row[0].force_encoding('utf-8')
    puts ['(4)', str.inspect, str[0].inspect, str.encoding].join("\t")
  end
end

conn.copy_data('COPY (select * from test_copy_to) TO STDOUT CSV') do
  while row = conn.get_copy_data
    row = row.chomp

    puts ['(5)', row.inspect, row[0].inspect, row.encoding].join("\t")
    str = row.force_encoding('utf-8')
    puts ['(6)', str[0].inspect, str.inspect, str.encoding].join("\t")
  end
end

str = conn.exec('select * from test_copy_to').getvalue(0, 0)
puts ['(7)', str.inspect, str[0].inspect, str.encoding].join("\t")

__END__
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-darwin18]

(1)     "Ä"     "\xC3"  UTF-8
(2)     "Ä"     "\xC3"  UTF-8
(3)     "\xC3\x84"      "\xC3"  ASCII-8BIT
(4)     "Ä"     "Ä"     UTF-8
(5)     "\xC3\x84"      "\xC3"  ASCII-8BIT
(6)     "Ä"     "Ä"     UTF-8
(7)     "Ä"     "Ä"     UTF-8