Creates the signable string for a response and returns it.

        Keyword arguments:
        request -- A request object which can be consumed by this API.
        authheaders -- A string-indexable object which contains the headers appropriate for this signature version.
        response_body -- A string or bytes-like object which represents the body of the response.

def signable(self, request, authheaders, response_body):
        """Creates the signable string for a response and returns it.

        Keyword arguments:
        request -- A request object which can be consumed by this API.
        authheaders -- A string-indexable object which contains the headers appropriate for this signature version.
        response_body -- A string or bytes-like object which represents the body of the response.
        """
        nonce = authheaders["nonce"]
        timestamp = request.get_header("x-authorization-timestamp")
        try:
            body_str = response_body.decode('utf-8')
        except:
            body_str = response_body
        return '{0}\n{1}\n{2}'.format(nonce, timestamp, body_str)