Fix filter body (again)

Correct kamailio's filter_body function, again, because it was still broken.

This time, kamailio wasn't correctly handling multipart MIME messages where the
content-type header within the MIME part came before other headers, since
kamailio expects that content-type will be the last and/or only header.

Specifically, messages that looked like this:

```
--boundary
Content-Type: application/sdp
Content-Description: Description_Sdp_Message_Body
Content-Disposition: session
```

The filtered body would still include the Content-Description and
Content-Disposition headers, despite that they aren't part of the message body.