Inheritance of BigDecimal breaks is_a? function in JRuby

jruby-1.6.7 1.8

~/tmp % jruby -v
jruby 1.6.7 (ruby-1.8.7-p357) (2012-02-22 3e82bc8) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_31) [darwin-x86_64-java]
~/tmp % jruby zz.rb
Decimal.new is a Decimal object?: false
SubString.new is a SubString object?: true

jruby-1.6.7 1.9

~/tmp % jruby -v
jruby 1.6.7 (ruby-1.8.7-p357) (2012-02-22 3e82bc8) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_31) [darwin-x86_64-java]
~/tmp % jruby --1.9 zz.rb
Decimal.new is a Decimal object?: false
SubString.new is a SubString object?: true

jruby-1.7.0-preview1

~/tmp % jruby -v
jruby 1.7.0.preview1 (ruby-1.9.3-p203) (2012-05-19 00c8c98) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_31) [darwin-x86_64-java]
~/tmp % jruby zz.rb
Decimal.new is a Decimal object?: false
SubString.new is a SubString object?: true

mri-1.9.3

~/tmp % ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin11.3.0]
~/tmp % ruby zz.rb
Decimal.new is a Decimal object?: true
SubString.new is a SubString object?: true

rbx-1.9.3

~/tmp % ruby -v
rubinius 2.0.0dev (1.8.7 4f53f0ce yyyy-mm-dd JI) [x86_64-apple-darwin11.3.0]
~/tmp % ruby zz.rb
Decimal.new is a Decimal object?: true
SubString.new is a SubString object?: true

zz.rb

require "bigdecimal"

class Decimal < BigDecimal
end

puts "Decimal.new is a Decimal object?: #{Decimal.new("9876543210").is_a?(Decimal)}"

class SubString < String
end

puts "SubString.new is a SubString object?: #{SubString.new("test").is_a?(SubString)}"