structure in twig template adds an extra space in classes fieldset

the template for fieldset has the following code
<fieldset class="{{ classes }} {% if collapsed %}collapsed{% endif %}">

this adds an annoying extra space after the last class, the culprit is the segment
{% if collapsed %}collapsed{% endif %}"
*note the initial space

besides there were no collapsed attribute, this approach inserts a space into the classes attribute in all cases

solution:
traverse the classes and inject afterwards the collapsed attribute if needed