visibility again.  we no longer provide __send, __send!.

* eval.c (rb_invoke_method): new method to honor private
  visibility.  if it's invoked in a function call style, it calls
  private methods as well (previous 1.9 send behavior).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@13824 b2dd03c8-39d4-4d8f-98ff-823fe69b080e