add a test for yaml ast roundtripping

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@35491 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/psych/test_parser.rb

@@ -32,6 +32,13 @@ module Psych
       @handler.parser = @parser
     end
 
+    def test_ast_roundtrip
+      parser = Psych.parser
+      parser.parse('null')
+      ast = parser.handler.root
+      assert_match(/^null/, ast.yaml)
+    end
+
     def test_exception_memory_leak
       yaml = <<-eoyaml
 %YAML 1.1