fix fd mask detection on os x

	This was broken in r59440 (3215b27a)
	[Bug #14248]

	From: Stefan Kaes <skaes@railsexpress.de>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@61582 b2dd03c8-39d4-4d8f-98ff-823fe69b080e