JRuby 1.7.1 vs MRI 1.9.3-p327 write binary string to file when Encoding.default_internal is UTF-8

JRuby 1.7.1 result

When Encoding.default_internal is nil
==============================
Wirte remote_binary_data to test_file 
Encoding.default_external:#<Encoding:UTF-8>
Encoding.default_internal:nil
data encoding: #<Encoding:ASCII-8BIT>
file external encoding: nil
file internal encoding: nil
file size:  13686

Wirte local_binary_data to test_file 
Encoding.default_external:#<Encoding:UTF-8>
Encoding.default_internal:nil
data encoding: #<Encoding:UTF-8>
file external encoding: nil
file internal encoding: nil
file size: 13686

When Encoding.default_internal is UTF-8
==============================
Wirte remote_binary_data to test_file 
Encoding.default_external:#<Encoding:UTF-8>
Encoding.default_internal:#<Encoding:UTF-8>
data encoding: #<Encoding:ASCII-8BIT>
file external encoding: #<Encoding:UTF-8>
file internal encoding: nil
file size:  20025

Wirte local_binary_data to test_file 
Encoding.default_external:#<Encoding:UTF-8>
Encoding.default_internal:#<Encoding:UTF-8>
data encoding: #<Encoding:UTF-8>
file external encoding: #<Encoding:UTF-8>
file internal encoding: nil
file size: #<Encoding::UndefinedConversionError: Input length = 1>

MRI 1.9.3-p327 result

When Encoding.default_internal is nil
==============================
Wirte remote_binary_data to test_file 
Encoding.default_external:#<Encoding:UTF-8>
Encoding.default_internal:nil
data encoding: #<Encoding:ASCII-8BIT>
file external encoding: nil
file internal encoding: nil
file size:  13686

Wirte local_binary_data to test_file 
Encoding.default_external:#<Encoding:UTF-8>
Encoding.default_internal:nil
data encoding: #<Encoding:UTF-8>
file external encoding: nil
file internal encoding: nil
file size: 13686

When Encoding.default_internal is UTF-8
==============================
Wirte remote_binary_data to test_file 
Encoding.default_external:#<Encoding:UTF-8>
Encoding.default_internal:#<Encoding:UTF-8>
data encoding: #<Encoding:ASCII-8BIT>
file external encoding: #<Encoding:UTF-8>
file internal encoding: nil
file size:  #<Encoding::UndefinedConversionError: "\x89" from ASCII-8BIT to UTF-8>

Wirte local_binary_data to test_file 
Encoding.default_external:#<Encoding:UTF-8>
Encoding.default_internal:#<Encoding:UTF-8>
data encoding: #<Encoding:UTF-8>
file external encoding: #<Encoding:UTF-8>
file internal encoding: nil
file size: 13686

test.rb

require 'open-uri'

Encoding.default_external=Encoding::UTF_8

remote_binary_data = open('https://github.com/apple-touch-icon-114.png').read
open('touch-icon-114.png','wb'){ |f| f.write(remote_binary_data)}
local_binary_data = open('touch-icon-114.png').read

def write_file(data)
  puts "Encoding.default_external:" + Encoding.default_external.inspect
  puts "Encoding.default_internal:" + Encoding.default_internal.inspect
  puts "data encoding: #{data.encoding.inspect}"
  begin
    open('test_file','w') do |f| 
      puts "file external encoding: #{f.external_encoding.inspect}"; 
      puts "file internal encoding: #{f.internal_encoding.inspect}"; 
      f.write(data)
    end 
  rescue  Exception => e
    e.inspect
  end 
end

puts "When Encoding.default_internal is nil"
puts "="*30

puts "Wirte remote_binary_data to test_file "
puts "file size:  #{write_file(remote_binary_data)}\n\n"

puts "Wirte local_binary_data to test_file "
puts "file size: #{write_file(local_binary_data)}"

puts ""

Encoding.default_internal=Encoding::UTF_8
puts "When Encoding.default_internal is UTF-8"
puts "="*30

puts "Wirte remote_binary_data to test_file "
puts "file size:  #{write_file(remote_binary_data)}\n\n"

puts "Wirte local_binary_data to test_file "
puts "file size: #{write_file(local_binary_data)}"