really a module, whose methods must be safe for reciever's type.

* eval.c (rb_eval): nosuper should not be inherited unless the
  overwritten method is an undef placeholder.

* parse.y (primary): allow 'when'-less case statement; persuaded
  by Sean Chittenden.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@3032 b2dd03c8-39d4-4d8f-98ff-823fe69b080e