Mail body and subject are not added

Because of the way Drupal handles mail, adding the subject and body in the params array of \Drupal\Core\Mail\MailManager::mail() method will not add them to the mail subject and body.

We need to use a hook_mail / hook_mail_alter functions to move the subject and body from the params array to the subject/body accordingly.

  /**
   * Implements hook_mail_alter() for the rules module.
   *
   * Extract subject and body from emails and set them for mailer.
   */
  function rules_mail_alter(&$message) {
    if ($message['module'] == 'rules') {
      $message['subject'] = !empty($message['params']['subject']) ? $message['params']['subject'] : '';
      if ($message['key'] === 'rules_action_mail_rules_send_email') {
        $message['body'] = !empty($message['params']['message']) ? array($message['params']['message']) : array();
      }
      else {
        $message['body'] = !empty($message['params']['body']) ? array($message['params']['body']) : array();
      }
    }
  }

This is of course a bug with drupal that will hopefully be fixed with 1346036.

The problem is that this issue is aimed at Drupal 8.2.x

I think it would be best to give a solution for this issue in the context of rules for 8.0.x and 8.1.x