Basic auth cache and SslBump fix

Basic auth cache and SslBump fix

This patch fixes the following bug:
 1) A user sends a CONNECT request with valid credentials
 2) Squid checks the credentials and adds the user to the user cache
 3) The same user sends a CONNECT request with invalid credentials
 4) Squid overwrites the entry in the user cache and denies the second
    CONNECT request
 5) The user sends a GET request on the first SSL connection which is
    established by now
 6) Squid knows that it does not need to check the credentials on the
    bumped connection but still somehow checks again whether the user is
    successfully authenticated
 7) Due to the second CONNECT request the user is regarded as not
    successfully authenticated
 8) Squid denies the GET request of the first SSL connection with 403
    ERR_CACHE_ACCESS_DENIED

On proxies with Basic authentication and SSL bumping, this can be used
to prevent a legitimate user from making any HTTPS requests

This is a Measurement Factory project