'checks for "Negotiate" in proper auth header'

def negotiate_value(self, headers):

    authreq = headers.get(self.auth_header, None)
    if authreq:
        rx = re.compile('(?:.*,)*\\s*Negotiate\\s*([^,]*),?', re.I)
        mo = rx.search(authreq)
        if mo:
            return mo.group(1)
        else:
            LOG.debug(('regex    failed    on:    %s' % authreq))
    else:
        LOG.debug(('%s    header    not    found' % self.auth_header))
    return None
