Allow use of DelegateClass in ractors

Tempfile uses DelegateClass and Tempfile should be able to be used
by different ractors.

Author: luke-gruber

lib/delegate.rb

@@ -345,10 +345,16 @@ def __setobj__(obj)
 end
 
 def Delegator.delegating_block(mid) # :nodoc:
-  lambda do |*args, &block|
+  prok = lambda do |*args, &block|
     target = self.__getobj__
     target.__send__(mid, *args, &block)
-  end.ruby2_keywords
+  end
+  prok.ruby2_keywords
+  if defined?(Ractor.shareable_proc)
+    Ractor.shareable_proc(&prok)
+  else
+    prok
+  end
 end
 
 #