merge send data with authorization data,
remove all undefined values
function sanitize(sender, data) {
  data = _.defaults({}, {
    client_id: sender.client_id, client_secret: sender.client_secret
  }, data);

  return _.reduce(data, function (acc, value, key) {
    if (! _.isUndefined(value)) {
      acc[key] = value;
    }
    return acc;
  }, {});
}