delegate.rb: keep special methods

* lib/delegate.rb (Delegator): keep source information methods
  which start and end with '__'.  [ruby-core:58572] [Bug #9155]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44630 b2dd03c8-39d4-4d8f-98ff-823fe69b080e