* ext/psych/lib/psych/visitors/to_ruby.rb: correctly register
  self-referential strings. Fixes tenderlove/psych #135

* test/psych/test_string.rb: appropriate test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40137 b2dd03c8-39d4-4d8f-98ff-823fe69b080e