* lib/rss/rss.rb (RSS::RootElementMixin#to_xml): added.
  [ruby-talk:197284]

  We can convert RSS version easily like the following:
    rss10 = RSS::Parser.parse(File.read("1.0.rdf"))
    File.open("2.0.rss", "w") {|f| f.print(rss10.to_xml("2.0"))}

* test/rss/test_1.0.rb: added #to_xml test.
* test/rss/test_2.0.rb: ditto.

* test/rss/rss-testcase.rb: added some helper methods that
  generates sample RSS 2.0.

* sample/rss/convert.rb: added a sample script to convert RSS format.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@10301 b2dd03c8-39d4-4d8f-98ff-823fe69b080e