Make charset header accessible, too

program/lib/Roundcube/rcube_message_header.php

@@ -195,6 +195,7 @@ class rcube_message_header
         'content-transfer-encoding' => 'encoding',
         'in-reply-to'               => 'in_reply_to',
         'content-type'              => 'ctype',
+        'charset'                   => 'charset',
         'references'                => 'references',
         'return-receipt-to'         => 'mdn_to',
         'disposition-notification-to' => 'mdn_to',