* lib/rexml/document.rb: add REXML::Document#document.
      reported by Tomas Hoger <thoger@redhat.com> and patched by nahi.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48402 b2dd03c8-39d4-4d8f-98ff-823fe69b080e