Fixed support for HTTP redirects on Windows

Also, since we didn't close the file handle in this case, and Windows
doesn't let you unlink open files, 0install actually reported a failure
to delete the temporary file instead of the real error. Fixed.

With --verbose, the error was:

info: Download failed: Invalid_argument("Unix.ftruncate not implemented")
Unix.Unix_error(Unix.EACCES, "unlink", "C:\\Users\\...\\0install-289dec-download")