Tempfiles created in current working directory only in Windows

[test-org-rename]

Reading the code shows that there is a special handling for Windows, the temp files are created in the current working directory. This is strange, because Tempfile::open() acts well on windows by creating the file in the configured temp directory. It may really be worse to create the file on the cwd because permissions may be restricted.
So I propose to remove this special handling (see graphviz.rb method output near line 367).