Do not fetch entire attached parts when rendering their pretty header

Do not fetch entire attached parts when rendering their pretty header

The code was falling back to the generic code path by accident; the
ENVELOPE fetching was complete nonsense as the data are provided by the
constructor.

Trying to fetch the List-Post and References headers through
TreeItemPart doesn't make any sense; the code was defaulting to fetching
the full attachment body just by accident. It also didn't provide any
useful data back anyway; that should be done by using our existing
header parser in this scenario. That's a future work, not something
covered by this patch -- this one simply prevents extra network traffic
from being triggered.

Thanks to Roberth Sjonøy <roberth.sjonoy@gmail.com> for bringing my
attention to this.

Change-Id: Ie9582b7de1dcf981bc71c970aa3206715cf83541