Issue #3423235 by Grimreaper: Accordion: Test default on boolean

Closes #3423235

templates/patterns/accordion/pattern-accordion.html.twig

@@ -2,7 +2,7 @@
   {% set attributes = attributes.addClass('accordion-' ~ variant|lower|replace({'_': '-'})) %}
 {% endif %}
 {% set accordion_id = accordion_id|default("accordion-" ~ random()) %}
-{% set keep_open = keep_open|default(false) ? 'true' : 'false' %}
+{% set keep_open = keep_open ? 'true' : 'false' %}
 <div{{ attributes.addClass('accordion').setAttribute('id', accordion_id).setAttribute('data-usb-keep-open', keep_open) }}>
   {{ content }}
 </div>