Really fix #1896. YAML class structure changed between 1.8.2 and 1.8.3,...

Really fix #1896.  YAML class structure changed between 1.8.2 and 1.8.3, breaking the original implementation.  Duck-type instead of checking kind_of?

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@2603 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activerecord/lib/active_record/fixtures.rb

@@ -308,7 +308,7 @@ def read_fixture_files
         # YAML fixtures
         begin
           if yaml = YAML::load(erb_render(IO.read(yaml_file_path)))
-            yaml = yaml.value if yaml.kind_of?(YAML::Syck::PrivateType)
+            yaml = yaml.value if yaml.respond_to?(:type_id) and yaml.respond_to?(:value)
             yaml.each do |name, data|
               self[name] = Fixture.new(data, @class_name)
             end