contact_mail() casts rendered markup to string

Problem/Motivation

<?php
$message['body'][] = (string) \Drupal::service('renderer')->renderPlain($build);
?>

That breaks sending HTML mails for example with swift mailer, which tries to be correct and escapes non-safe HTML.

Proposed resolution

Remove the string casts.

Remaining tasks

User interface changes

API changes

Data model changes