Corrected wrong call to Mailer sendmessage

Zikula Mailer sendmessage requires boolean type for parameter 'html', as seen from this code fragment from system/Mailer/lib/Mailer/Api/User.php:
```
if (isset($args['html']) && is_bool($args['html'])) {
    $mail->IsHTML($args['html']); // set email format to HTML
} else {
    $mail->IsHTML($this->getVar('html')); // set email format to the default
}
```
Proposed changes in this PR coorects the wrong call.

Also, in case of plane text message format, the message body is treated as required.