* ext/psych/lib/psych/visitors/yaml_tree.rb: dumping strings with
  quotes should not have changed.  [ruby-core:59316] [Bug #9300]

* ext/psych/lib/psych.rb: fixed missing require.

* test/psych/test_string.rb: test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44531 b2dd03c8-39d4-4d8f-98ff-823fe69b080e