Define Set#to_set so that aSet.to_set returns self.

* lib/set.rb (Set#to_set): Define Set#to_set so that aSet.to_set
  returns self. [Fixes GH-359]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42051 b2dd03c8-39d4-4d8f-98ff-823fe69b080e