merge revision(s) 44531: [Backport #9300]

	* 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/branches/ruby_2_1@45079 b2dd03c8-39d4-4d8f-98ff-823fe69b080e