Ruby 1.9.3 standard YAML makes me a sad panda

yaml-example.rb

1.9.3p125 :004 > YAML::VERSION
 => "1.2.2" 
1.9.3p125 :005 > a = "foo"
 => "foo" 
1.9.3p125 :007 > a.force_encoding("BINARY")
 => "foo" 
1.9.3p125 :008 > a.encoding
 => #<Encoding:ASCII-8BIT> 
1.9.3p125 :009 > YAML.dump(a)
 => "--- !binary |-\n  Zm9v\n" 
1.9.3p125 :010 > exit
 mazikeen:./.msf4$ rvm use default
Using /home/todb/.rvm/gems/ruby-1.9.1-p378
 mazikeen:./.msf4$ irb
1.9.1p378 :002 > require 'yaml'
 => true 
1.9.1p378 :003 > a = "foo"
 => "foo" 
1.9.1p378 :004 > a.force_encoding("BINARY")
 => "foo" 
1.9.1p378 :005 > a.encoding
 => #<Encoding:ASCII-8BIT> 
1.9.1p378 :006 > YAML.dump(a)
 => "--- foo\n" 
1.9.1p378 :007 > exit