`@res` is an Array, so `@res =~ /\n\z/` calls `Object#=~` which always
returns nil.
I guess it should be `@res.last =~ /\n\z/`, but the change causes test
failures.

This bug was found during work for removal of `Object#=~`.
[Feature #15231]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65985 b2dd03c8-39d4-4d8f-98ff-823fe69b080e