Based on patch by akr [ruby-core:88477], use Tempfile.create
to avoid unnecessary unlink call.  Unlike akr's original patch,
this does not change the return value of flush.

Thanks-to: Tanaka Akira <akr@fsij.org>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64363 b2dd03c8-39d4-4d8f-98ff-823fe69b080e