may be passed to the Psych::Coder object.
* ext/psych/lib/psych/visitors/yaml_tree.rb: support for visiting
  arbitrary objects set on the coder.
* test/psych/test_coder.rb: supporting test case.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@30609 b2dd03c8-39d4-4d8f-98ff-823fe69b080e