HTTP: don't abort connections with pending Negotiate authentication

... similarly to how NTLM works as Negotiate is in fact often NTLM with
another name.
HTTP: Free (proxy)userpwd for NTLM/Negotiate after sending a request

Sending NTLM/Negotiate header again after successful authentication
breaks the connection with certain Proxies and request types (POST to MS
Forefront).
http_perhapsrewind: don't abort CONNECT requests

...they never have a body