If the system receives an email by a collaborator which has not yet been
added to the ticket (a friend of a friend — that is, a collaborator forwards
an email to a third-party), a header is added to the thread body something
like:

Received From: afriendofafriend@mycompany.tld

However, if the thread body is text and the HTML ticket thread is enabled,
then the text formatting hint will be lost and the body will be assumed as
HTML deeper inside the thread entry creation process. Therefore, the
whitespace inside the resulting thread entry will be collapsed.

This patch addresses the issue by maintaining the original format hint with
the thread body.