Issue #3541862 by avpaderno, smustgrave: The output shown for example code is wrong

 *  $attributes['class'] = ['black-cat', 'white-cat'];

 *  $attributes['class'][] = 'black-white-cat';

 *  echo '<cat class="cat ' . $attributes['class'] . '"' . $attributes . '>';

 *  // Produces <cat class="cat black-cat white-cat black-white-cat" id="socks" class="cat black-cat white-cat black-white-cat">

 *  // Produces <cat class="cat black-cat white-cat black-white-cat" id="socks" class="black-cat white-cat black-white-cat">

 * @endcode

 *

 * When printing out individual attributes to customize them within a Twig

 * @endcode

 * Produces:

 * @code

 * <cat class="cat black-cat white-cat black-white-cat my-custom-class" id="socks">

 * <cat class="black-cat white-cat black-white-cat my-custom-class" id="socks">

 * @endcode

 *

 * The attribute keys and values are automatically escaped for output with