merges r30475 from trunk into ruby_1_9_2.

--
* ext/psych/lib/psych/json/tree_builder.rb (start_mapping): tags
  should not be included in JSON mapping

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_2@30566 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/psych/test_coder.rb

@@ -89,6 +89,10 @@ module Psych
       end
     end
 
+    def test_json_dump_exclude_tag
+      refute_match('TestCoder::InitApi', Psych.to_json(InitApi.new))
+    end
+
     def test_map_takes_block
       coder = Psych::Coder.new 'foo'
       tag = coder.tag