runtime: Http Negotiate authentication fails on Unix

@ManuelAVR reported an issue where .NET Core fails to authenticate against his proxy server.

The proxy server is sending:

< Proxy-Authenticate: Negotiate
< Proxy-Authenticate: NTLM

.NET Core doesn’t reply to the proxy server to authenticate. It returns the 407 as the response result.

Trying to authenticate with curl using --proxy-negotiate fails with: gss_init_sec_context() failed: SPNEGO cannot find mechanisms to negotiate. Using --proxy-ntlm works.

There is no Kerberos ticket. Proxy server and credentials are provided using http_proxy and https_proxy envvars.

Perhaps .NET Core gets the same error from gss_init_sec_context and then stops. Maybe at that point it should try NTLM auth instead?

It doesn’t work either when using the curl httphandler. Probably curl also has this issue: https://github.com/curl/curl/issues/876.

CC @wfurt