httpClient = (options) => {
  const { fetch, http } = options;

  // @TODO fetch should be removed, and http used instead
  // provide a default fetch implementation
  return fetch || http || Http;
}
Swagger-UI uses baseDoc instead of url, this helper function exists
to allow both.

In browser environment, we allow to pass a relative URI Reference,
and we resolve it against the document's baseURI before passing it deeper
to swagger-client code.