Sitemap generation missing xmlns:image and xmlns:xhtml

<?php

$loader = require_once __DIR__ . '/../vendor/autoload.php';

$urlSet = new Thepixeldeveloper\Sitemap\Urlset();

$urlA = (new Thepixeldeveloper\Sitemap\Url('http://www.example.com/en/'))
    ->setLastMod(date(DATE_W3C, time()))
    ->setChangeFreq('weekly')
    ->setPriority(0.5);

$urlB = (new Thepixeldeveloper\Sitemap\Url('http://www.dummy.com/'))
    ->setLastMod(date(DATE_W3C, time()))
    ->setChangeFreq('weekly')
    ->setPriority(0.2);

$urlB
    ->addSubElement(new \Thepixeldeveloper\Sitemap\Subelements\Link('en', 'http://www.example.com/en/'))
    ->addSubElement(new \Thepixeldeveloper\Sitemap\Subelements\Link('de', 'http://www.example.com/de/'))
    ->addSubElement(new \Thepixeldeveloper\Sitemap\Subelements\Image('http://goo.gl/BwuUJ5'))
;

$urlSet->addUrl($urlA);
$urlSet->addUrl($urlB);


echo (new \Thepixeldeveloper\Sitemap\Output())->getOutput($urlSet);