Do not JSON encode stringable values

[jderusse]

When using stimulus_controller with values generated by twig, the values are converted to JSON. Which prevent the following code working out of the box.

{% set tooltip %}
  Hello <strong>{{ name }}</strong>
{% endset %}

<div {{ stimulus_controller('tooltip', {html: true, title: tooltip}) }}>
  title
</div>

Because the content of the variable tooltip is an \Twig\Markup object.

This PR stringifies values instead of json_encoding.

[weaverryan]

That's a nice catch - thank you Jérémy!