http: don't enable chunked during authentication negotiations

As mentioned in bug report #2956968, the HTTP code wouldn't send the
first empty chunk during the auth negotiation phase of the HTTP request
sending, so the server would wait for data to come and libcurl would
wait for data to arrive... I've made the code not enable chunked
encoding until the auth negotiation is done and thus this scenario
doesn't occur anymore.

Reported by: Sidney San Martín
Bug: http://curl.haxx.se/bug/view.cgi?id=2956968