merge revision(s) 42906: [Backport #8891]

	* thread.c (rb_mutex_unlock): Mutex#unlock no longer raise
	  an exception even if uses on trap. [Bug #8891]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@42926 b2dd03c8-39d4-4d8f-98ff-823fe69b080e