ISO-8859-encoded subject not decoded when forwarding in webmail

If I try in the webapp to forward an email that has an ISO-8859-encoded subject, it appears un-decoded in the message body, like this:

```
---- Forwarded Message ----
Subject:  =?iso-8859-1?B?hucdrDucllohclGDceiEloiucgDCGIo=?=
Date:     ...
From:     ...
To:       ...

...
```