* ext/psych/lib/psych/visitors/yaml_tree.rb: register time objects so
  they are referenced as ids during output.
* test/psych/test_date_time.rb: corresponding test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41804 b2dd03c8-39d4-4d8f-98ff-823fe69b080e