Ates Goral identified a problem in http.c:add_buffer_send() when a debug

callback was used, as it could wrongly pass on a bad size for the outgoing
HTTP header. The bad size would be a very large value as it was a wrapped
size_t content. This happened when the whole HTTP request failed to get sent
in one single send.  http://curl.haxx.se/mail/lib-2007-11/0165.html