delegate.rb: ignore unset target

* lib/delegate.rb (Delegator#method_missing): ignore the target if not
  set, and delegate to global methods.  [ruby-core:58572] [Bug #9155]
* lib/delegate.rb (Delegator#respond_to_missing): ditto.
* lib/delegate.rb (SimpleDelegator#__getobj__): yield and return if
  not delegated but a block is given, like as Hash#fetch.
* lib/delegate.rb (DelegateClass#__getobj__): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43984 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-12-04

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43985 b2dd03c8-39d4-4d8f-98ff-823fe69b080e