* ext/psych/lib/psych/visitors/to_ruby.rb: raise an exception when
deserializing an alias that does not exist.
* test/psych/test_merge_keys.rb: corresponding test.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@32578 b2dd03c8-39d4-4d8f-98ff-823fe69b080e