Allow "--- \n" as a yaml dumped result for nil.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@32173 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/psych/test_nil.rb

@@ -4,7 +4,7 @@ module Psych
   class TestNil < TestCase
     def test_nil
       yml = Psych.dump nil
-      assert_equal "--- \n...\n", yml
+      assert_match /--- \n(?:\.\.\.\n)?/, yml
       assert_equal nil, Psych.load(yml)
     end