better assertion
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56409 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
better assertion
assert_kind_of for better assertion message.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56409 b2dd03c8-39d4-4d8f-98ff-823fe69b080e