HTTP auth: fix proxy Negotiate bug

If a proxy offers several Authentication schemes where NTLM and
Negotiate are offered by the proxy and you tell libcurl not to use the
Negotiate scheme then the request never returns when the proxy answers
with its HTTP 407 reply.

It is reproducible by the following steps:

- Use a proxy that offers NTLM and Negotiate ( CURLOPT_PROXY and
CURLOPT_PROXYPORT )

- Tell libcurl NOT to use Negotiate CURL_EASY_SETOPT(CURLOPT_PROXYAUTH,
CURLAUTH_BASIC | CURLAUTH_DIGEST | CURLAUTH_NTLM )

- Start the request

The call to CURL_EASY_PERFORM never returns. If you switch on debug
logging you can see that libcurl issues a new request As soon as it
received the 407 reply. Instead it should return and set the response
code to 407.

Bug: http://curl.haxx.se/mail/lib-2011-10/0323.html