http_perhapsrewind: consider NTLM over proxy too

The logic previously checked for a started NTLM negotiation only for
host and not also with proxy, leading to problems doing POSTs over a
proxy NTLM that are larger than 2000 bytes. Now it includes proxy in the
check.

Bug: http://curl.haxx.se/bug/view.cgi?id=3582321
Reported by: John Suprock