Prepare, return the required HTTP headers.

        Base 64 encode the parameters, sign it with the secret key,
        create the HTTP headers, return the whole payload.

        Arguments:
        params -- a dictionary of parameters

def prepare(self, params):
        """
        Prepare, return the required HTTP headers.

        Base 64 encode the parameters, sign it with the secret key,
        create the HTTP headers, return the whole payload.

        Arguments:
        params -- a dictionary of parameters
        """
        jsonparams = json.dumps(params)
        payload = base64.b64encode(jsonparams.encode())
        signature = hmac.new(self.secret_key.encode(), payload,
                             hashlib.sha384).hexdigest()

        return {'X-GEMINI-APIKEY': self.api_key,
                'X-GEMINI-PAYLOAD': payload,
                'X-GEMINI-SIGNATURE': signature}