Fold the To list passed to mail()

mailSend() joined the To addresses for mail() with its own implode, so
a long list still went out as one line, while the To header used for
getSentMIMEMessage() and DKIM signing is folded by addrAppend(). mail()
keeps CRLF + space folds in its to argument, so build it with
addrAppend() too.

src/PHPMailer.php

@@ -2090,7 +2090,8 @@ class PHPMailer
         foreach ($this->to as $toaddr) {
             $toArr[] = $this->addrFormat($toaddr);
         }
-        $to = trim(implode(', ', $toArr));
+        //Fold long lists the same way as the To header, mail() keeps CRLF + WSP folds intact
+        $to = trim(substr($this->addrAppend('To', $this->to), strlen('To:')));
 
         //If there are no To-addresses (e.g. when sending only to BCC-addresses)
         //the following should be added to get a correct DKIM-signature.