As reported by "Tro" in http://curl.haxx.se/mail/lib-2007-06/0161.html and

http://curl.haxx.se/mail/lib-2007-06/0238.html, libcurl didn't properly do
no-body requests on FTP files on re-used connections properly, or at least
it didn't provide the info back in the header callback properly in the
subsequent requests.