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_1@45104 b2dd03c8-39d4-4d8f-98ff-823fe69b080e