SSML not working

[askinucuncu]

$text = \Dialogflow\RichMessage\Text::create()
->text('This is text')
->ssml('This is ssml')
;
$agent->reply($text);

output data

{
"fulfillmentMessages": [
{
"text": {
"text": [
"This is text"
]
}
}
],
"outputContexts": []
}