code: public OauthData refreshOauthData(String refreshToken) throws HelloSignException {
          OauthData data = new OauthData(
                  httpClient.withAuth(auth).withPostField(OAUTH_GRANT_TYPE, OAUTH_GRANT_TYPE_REFRESH_TOKEN)
                          .withPostField(OAUTH_REFRESH_TOKEN, refreshToken).post(OAUTH_TOKEN_URL).asJson());
          if (data != null) {
              setAccessToken(data.getAccessToken(), data.getTokenType());
          }
          return data;
      }
docs: Refreshes the OauthData for this client with the provided refresh token.
  
  @param refreshToken String
  @return OauthData new OAuthData returned from HelloSign
  @throws HelloSignException thrown if there's a problem processing the
  HTTP request or the JSON response.