* ext/psych/lib/psych/nodes/node.rb: Rename `to_yaml` to just `yaml`
in order to avoid YAML::ENGINE switching from replacing this method.
* test/psych/helper.rb: fix tests for method name change.
* test/psych/test_document.rb: ditto
* test/psych/visitors/test_emitter.rb: ditto
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@33384 b2dd03c8-39d4-4d8f-98ff-823fe69b080e