* ext/psych/lib/psych/visitors/to_ruby.rb: merge key values that
  contain something besides a hash should be left in tact.

* test/psych/test_merge_keys.rb: test for change

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38788 b2dd03c8-39d4-4d8f-98ff-823fe69b080e