IronRuby doesn't respect encoding comments when eval'ing

encoding.rb

eval <<ENC
  # coding: utf-8
  def method
   puts "".encoding.inspect
  end
ENC

eval <<ENC
  # coding: big5
  def method2
    puts "".encoding.inspect
  end
ENC

eval <<ENC
  # coding: shift_jis
  def method3
    puts "".encoding.inspect
  end
ENC

method
method2
method3

output.sh

$ ruby encoding.rb
#<Encoding:UTF-8>
#<Encoding:Big5>
#<Encoding:Shift_JIS>

$ ir encoding.rb
#<Encoding:US-ASCII>
#<Encoding:US-ASCII>
#<Encoding:US-ASCII>