Reverting 84e3e3fe makes this go away

.gitignore

/gz18.txt
/gz19.txt

gz.rb

require 'rubygems'
require 'rubygems/format'

File.open('rack-1.4.1.gem', 'r') do |rack|
  Gem::Package.open(rack, 'r', nil) do |pkg|
    pkg.metadata

    pkg.each do |entry|
      print "#{entry.full_name} #{entry.header.size} #{entry.header.mode}"
      str = entry.read
      # print " #{str.encoding}" if str.respond_to?(:encoding)
      puts
    end
  end
end

output.txt

$ rake
jruby --1.8 -v
jruby 1.7.0.dev (ruby-1.8.7-p357) (2012-02-02 6971c33) (Java HotSpot(TM) Client VM 1.6.0_29) [darwin-i386-java]
jruby --1.8 -v gz.rb > gz18.txt
jruby --1.9 -v
jruby 1.7.0.dev (ruby-1.9.3-p28) (2012-02-02 6971c33) (Java HotSpot(TM) Client VM 1.6.0_29) [darwin-i386-java]
jruby --1.9 -v gz.rb > gz19.txt
diff -q gz18.txt gz19.txt
Files gz18.txt and gz19.txt differ
rake aborted!
Command failed with status (1): [diff -q gz18.txt gz19.txt...]

Tasks: TOP => default
(See full trace by running task with --trace)

rack-1.4.1.gem

Rakefile

task :default do
  sh "jruby --1.8 -v"
  sh "jruby --1.8 gz.rb > gz18.txt"
  sh "jruby --1.9 -v"
  sh "jruby --1.9 gz.rb > gz19.txt"
  sh "diff -q gz18.txt gz19.txt"
end

Reverting jruby/jruby@84e3e3fe makes this test succeed again.