OTRS/otrs — pull request #65 by reneeb (2014-03-07 14:20:23+00:00)
Title: do not substitute when content-id is empty

When Content-ID is just "<>" the result is really strange:

```
--=_mixed 0123456789012345_=
Content-Transfer-Encoding: quoted-printable
CContent-ID: <>

Link: http://www.invalid
```

is shown as

Links: http://www.invalid