nntp: ensure Message-ID is not folded for leafnode

nntp: ensure Message-ID is not folded for leafnode

Leafnode cannot handle Message-ID headers which are too long and
require folding via Email::Simple::Header.  Since there are
already many of these messages in git with the header already
folded, we need to handle the unfolding when emitting the
message via NNTP.

As far as we know, Leafnode is the only client software
incapable of handling this case.