StringIO#ungetc issue on JRuby 1.6.6 and 1.6.7 in 1.8.7 mode

The following code works and returns the root node on any version of JRuby prior to 1.6.6:

require 'stringio'
require 'rexml/document'
str = "<?xml version=\"1.0\"?>\n<xdoc>\n\t<errorcode>0</errorcode>\n</xdoc>"
data = StringIO.new(str)

char = data.getc
data.ungetc(char)

REXML::Document.new(data).root

As of JRuby 1.6.6 the last line above returns nil.

Below is the output from JRuby 1.6.5:

jruby -v -S irb
jruby 1.6.5.1 (ruby-1.8.7-p330) (2011-12-27 1bf37c2) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_29) [darwin-x86_64-java]
>> require 'stringio'
=> true
>> require 'rexml/document'
=> true
>> str = "<?xml version=\"1.0\"?>\n<xdoc>\n\t<errorcode>0</errorcode>\n</xdoc>"
=> "<?xml version="1.0"?>\n<xdoc>\n\t<errorcode>0</errorcode>\n</xdoc>"
>> data = StringIO.new(str)
=> #<StringIO:0x28c13406>
>>
?> char = data.getc
=> 60
>> data.ungetc(char)
=> nil
>>
?> REXML::Document.new(data).root
=> xdoc ....

On JRuby 1.6.6 (similarly for JRuby 1.6.7) :

jruby -v -S irb
jruby 1.6.6 (ruby-1.8.7-p357) (2012-01-30 5673572) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_29) [darwin-x86_64-java]
>> require 'stringio'
=> true
>> require 'rexml/document'
=> true
>> str = "<?xml version=\"1.0\"?>\n<xdoc>\n\t<errorcode>0</errorcode>\n</xdoc>"
=> "<?xml version="1.0"?>\n<xdoc>\n\t<errorcode>0</errorcode>\n</xdoc>"
>> data = StringIO.new(str)
=> #<StringIO:0x539ff66f>
>>
?> char = data.getc
=> 60
>> data.ungetc(char)
=> nil
>>
?> REXML::Document.new(data).root
=> nil

Java version output:

java -version
java version "1.6.0_29"
Java(TM) SE Runtime Environment (build 1.6.0_29-b11-402-11D50)
Java HotSpot(TM) 64-Bit Server VM (build 20.4-b02-402, mixed mode)

The above code is a subset of the code in Rails 3.0.10 to reproduce the issue in a standalone environment. It is occurring under ActiveSupport::XmlMini.parse which is using REXML by default