* lib/set.rb (Set#replace): Check if an object given is enumerable
  before clearing self.  Reported by yui-knk. [GH-675]
  https://github.com/ruby/ruby/pull/675

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47085 b2dd03c8-39d4-4d8f-98ff-823fe69b080e