merge revision(s) 44595: [Backport #9342]

* ext/thread/thread.c (rb_szqueue_clear): notify SZQUEUE_WAITERS
	  on SizedQueue#clear. [ruby-core:59462] [Bug #9342]

	* test/thread/test_queue.rb: add test. the patch is from
	  Justin Collins.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@44899 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/thread.rb

@@ -318,6 +318,18 @@ class SizedQueue < Queue
     end
   end
 
+  #
+  # Removes all objects from the queue.
+  #
+  def clear
+    super
+    @mutex.synchronize do
+      @max.times do
+        @enque_cond.signal
+      end
+    end
+  end
+
   #
   # Alias of push
   #