http: fix missing 'Content-Length: 0' while negotiating auth

- While negotiating auth during PUT/POST if a user-specified
  Content-Length header is set send 'Content-Length: 0'.

This is what we do already in HTTPREQ_POST_FORM and what we did in the
HTTPREQ_POST case (regression since afd288b2).

Prior to this change no Content-Length header would be sent in such a
case.

Bug: https://curl.haxx.se/mail/lib-2017-02/0006.html
Reported-by: Dominik Hölzl

Closes https://github.com/curl/curl/pull/1242