make the plain text footer standard compliant and add a space after '--'. It also adds a line break in front so that there is some spacing between the mail body and the footer

Signed-off-by: Bjoern Schiessle <bjoern@schiessle.org>

lib/private/Mail/EMailTemplate.php

@@ -461,7 +461,7 @@ EOF;
 
 		$this->htmlBody .= vsprintf($this->footer, [$text]);
 		$this->htmlBody .= $this->tail;
-		$this->plainBody .= '--' . PHP_EOL;
+		$this->plainBody .= PHP_EOL . '-- ' . PHP_EOL;
 		$this->plainBody .= str_replace('<br>', PHP_EOL, $text);
 	}