msg_iter: split_quotes adds trailing "\n"

msg_iter: split_quotes adds trailing "\n"

The regexp in split_quotes relies on the presence of a
final "\n", so add it wherever we need to instead of
making it the responsibility of every caller.

This probably doesn't matter in practice since every
email seems to have a "\n" as the final byte (due to
the way SMTP works), but maybe there's some odd ones
that'll get imported via lei.