fixed a missing require that causes trouble when using AS in a non-rails env.

activesupport/lib/active_support/xml_mini/libxml.rb

@@ -1,5 +1,6 @@
 require 'libxml'
 require 'active_support/core_ext/object/blank'
+require 'stringio'
 
 # = XmlMini LibXML implementation
 module ActiveSupport

activesupport/lib/active_support/xml_mini/libxmlsax.rb

@@ -1,5 +1,6 @@
 require 'libxml'
 require 'active_support/core_ext/object/blank'
+require 'stringio'
 
 # = XmlMini LibXML implementation using a SAX-based parser
 module ActiveSupport

activesupport/lib/active_support/xml_mini/nokogiri.rb

@@ -5,6 +5,7 @@ rescue LoadError => e
   raise e
 end
 require 'active_support/core_ext/object/blank'
+require 'stringio'
 
 # = XmlMini Nokogiri implementation
 module ActiveSupport

activesupport/lib/active_support/xml_mini/nokogirisax.rb

@@ -5,6 +5,7 @@ rescue LoadError => e
   raise e
 end
 require 'active_support/core_ext/object/blank'
+require 'stringio'
 
 # = XmlMini Nokogiri implementation using a SAX-based parser
 module ActiveSupport

activesupport/lib/active_support/xml_mini/rexml.rb

@@ -1,5 +1,6 @@
 require 'active_support/core_ext/kernel/reporting'
 require 'active_support/core_ext/object/blank'
+require 'stringio'
 
 # = XmlMini ReXML implementation
 module ActiveSupport