Encode mail subject properly

Sources/Subs-Post.php

@@ -363,7 +363,7 @@ function sendmail($to, $subject, $message, $from = null, $send_html = false)
 
 	// We don't need to mess with the subject line if no special characters were in it..
 	if (!empty($i18n_char))
-		$subject = '=?' . $txt['lang_character_set'] . '?Q?' . $subject . '?=';
+		$subject = mb_encode_mimeheader($subject, $['lang_character_set'],'Q',"\n");
 
 	// SMTP or sendmail?
 	if ($modSettings['mail_type'] == 'sendmail')
@@ -989,4 +989,4 @@ function sendNotifications($ID_TOPIC, $type)
 				AND ID_MEMBER != $ID_MEMBER", __FILE__, __LINE__);
 }
 
-?>
+?>