Ruby 1.9 encoding: Array#pack('Z*') is not binary

rbx-encoding-pack-zstar.out

rbx-2.0.0-dev
Running with rubinius 2.0.0dev (1.9.3 9ad741f3 yyyy-mm-dd JI) [i686-pc-linux-gnu]
* A binary packet
ASCII-8BIT: � ("\xFF") 1 bytes
* A unicode string
UTF-8: Václav Havel ("V\xC3\xA1clav Havel") 13 bytes
* Plan B
** Adding NUL
UTF-8: Václav Havel ("V\xC3\xA1clav Havel\x00") 14 bytes
** Forcing 8-bit
ASCII-8BIT: Václav Havel ("V\xC3\xA1clav Havel\x00") 14 bytes
** Putting them together with String#+
ASCII-8BIT: �Václav Havel ("\xFFV\xC3\xA1clav Havel\x00") 15 bytes
* Plan A
** Converting that string with pack("Z*"):
US-ASCII: Václav Havel ("V\xC3\xA1clav Havel\x00") 14 bytes
** Putting them together with String#+
An exception occurred running rbx-encoding-pack-zstar.rb
    undefined conversion for '"\xFF"' from ASCII-8BIT to US-ASCII (Rubinius::EncodingClass::Encoding::CompatibilityError)

Backtrace:
  Rubinius::Type.compatible_encoding at kernel/common/type19.rb:47
                           String#<< at kernel/common/string19.rb:420
                            String#+ at kernel/common/string.rb:65
                   Object#__script__ at rbx-encoding-pack-zstar.rb:36
    Rubinius::CodeLoader#load_script at kernel/delta/codeloader.rb:68
    Rubinius::CodeLoader.load_script at kernel/delta/codeloader.rb:110
             Rubinius::Loader#script at kernel/loader.rb:614
               Rubinius::Loader#main at kernel/loader.rb:815

1.9.3-p194
Running with ruby 1.9.3p194 (2012-04-20 revision 35410) [i686-linux]
* A binary packet
ASCII-8BIT: � ("\xFF") 1 bytes
* A unicode string
UTF-8: Václav Havel ("Václav Havel") 13 bytes
* Plan B
** Adding NUL
UTF-8: Václav Havel ("Václav Havel\u0000") 14 bytes
** Forcing 8-bit
ASCII-8BIT: Václav Havel ("V\xC3\xA1clav Havel\x00") 14 bytes
** Putting them together with String#+
ASCII-8BIT: �Václav Havel ("\xFFV\xC3\xA1clav Havel\x00") 15 bytes
* Plan A
** Converting that string with pack("Z*"):
ASCII-8BIT: Václav Havel ("V\xC3\xA1clav Havel\x00") 14 bytes
** Putting them together with String#+
ASCII-8BIT: �Václav Havel ("\xFFV\xC3\xA1clav Havel\x00") 15 bytes

jruby-1.6.7
Running with jruby 1.6.7 (ruby-1.9.2-p312) (2012-02-22 3e82bc8) (Java HotSpot(TM) Client VM 1.7.0_07) [linux-i386-java]
* A binary packet
ASCII-8BIT: � ("\u00FF") 1 bytes
* A unicode string
UTF-8: Václav Havel ("Václav Havel") 13 bytes
* Plan B
** Adding NUL
UTF-8: Václav Havel ("Václav Havel\u0000") 14 bytes
** Forcing 8-bit
ASCII-8BIT: Václav Havel ("V\u00C3\u00A1clav Havel\u0000") 14 bytes
** Putting them together with String#+
ASCII-8BIT: �Václav Havel ("\u00FFV\u00C3\u00A1clav Havel\u0000") 15 bytes
* Plan A
** Converting that string with pack("Z*"):
ASCII-8BIT: Václav Havel ("V\u00C3\u00A1clav Havel\u0000") 14 bytes
** Putting them together with String#+
ASCII-8BIT: �Václav Havel ("\u00FFV\u00C3\u00A1clav Havel\u0000") 15 bytes

rbx-encoding-pack-zstar.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

puts "Running with #{RUBY_DESCRIPTION}"

def dump(s)
  puts "#{s.encoding}: #{s} (#{s.inspect}) #{s.bytesize} bytes"
end

puts '* A binary packet'
binary = 255.chr
dump binary

puts '* A unicode string'
unicode = 'Václav Havel'
dump unicode

puts '* Plan B'
puts '** Adding NUL'
packed_b = unicode + 0.chr
dump packed_b
puts '** Forcing 8-bit'
packed_b.force_encoding('ASCII-8BIT')
dump packed_b

puts '** Putting them together with String#+'
together_b = binary + packed_b
dump together_b

puts '* Plan A'
puts '** Converting that string with pack("Z*"):'
packed_a = [unicode].pack('Z*')
dump packed_a # US-ASCII, WTF

puts '** Putting them together with String#+'
together_a = binary + packed_a
dump together_a