Regexp instead of string comparison for message/rfc822

Some mail clients (e.g. Thunderbird) add an extra ``name`` field to the ``Content-Type`` header when attaching an ``.eml`` file.

In my case, the header then was something like this..

```
Content-Type: message/rfc822;
  name="ForwardedMessage.eml";
```
..instead of
```
Content-Type: message/rfc822;
```

This led to problems when trying to get all attachments recurisvely through the ``Mail::AttachmentList`` class.

Luckily, the solution was pretty simple: Instead of doing a hard string comparison when checking the ``Content-Type`` header of a part, I changed it to use a regular expression instead. Also adjusted the fixture mail to ensure that its actually tested.