Attempts to authenticate the user if a token was provided.

def authenticate(self):
        """Attempts to authenticate the user if a token was provided."""
        if request.headers.get('Authorization', '').startswith('Negotiate '):
            in_token = base64.b64decode(request.headers['Authorization'][10:])

            try:
                creds = current_app.extensions['gssapi']['creds']
            except KeyError:
                raise RuntimeError('flask-gssapi not configured for this app')

            ctx = gssapi.SecurityContext(creds=creds, usage='accept')

            out_token = ctx.step(in_token)

            if ctx.complete:
                username = ctx._inquire(initiator_name=True).initiator_name
                return str(username), out_token

        return None, None