merge revision(s) 48905: [Backport #10616]

	* lib/tmpdir.rb (Dir.mktmpdir): Accept nil again, as Ruby 2.1.
	  [ruby-core:66943] [Bug #10616] Fixed by Alex Slynko.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@48947 b2dd03c8-39d4-4d8f-98ff-823fe69b080e