Support waking up blocking pop in Puma::QueueClose

Ruby 2.2 support for drain_on_shutdown test.

lib/puma/queue_close.rb

@@ -5,22 +5,22 @@ module Puma
   # Add a simple implementation for earlier Ruby versions.
   #
   module QueueClose
-    def initialize
-      @closed = false
-      super
-    end
     def close
+      num_waiting.times {push nil}
       @closed = true
     end
     def closed?
-      @closed
+      @closed ||= false
     end
     def push(object)
-      @closed ||= false
-      raise ClosedQueueError if @closed
+      raise ClosedQueueError if closed?
       super
     end
     alias << push
+    def pop(non_block=false)
+      return nil if !non_block && closed? && empty?
+      super
+    end
   end
   ::Queue.prepend QueueClose
 end