default_headers
Generate default headers
@private
@return {JSON}
function default_headers(class_context) {
  class_context = class_context || {};
  let headers   = {};

  if (class_context.token) {
    headers.Token = class_context.token;
  }

  if (!isBrowser) {
    headers['User-Agent'] = `Tago-Nodelib-${pkg.version}`;
  }

  return headers;
}