* ext/psych/lib/psych/visitors/yaml_tree.rb: fix symbol keys in coder
  emission.  Thanks @tjwallace
* test/psych/test_coder.rb: test for change

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40142 b2dd03c8-39d4-4d8f-98ff-823fe69b080e