io.c: copy_stream to duplex IO
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56416 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
io.c: copy_stream to duplex IO
http://twitter.com/knu/status/786505317974585344
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56416 b2dd03c8-39d4-4d8f-98ff-823fe69b080e