Normally it happens in addr_cmp function, but update_gossip_peerstates
forms a HashSet of addresses, so they should be lowercased beforehand.
Also adjust the mail_with_cc.txt to test for non-lowercase addresses.
we change the name on the wire as the old Chat-Group-Image header
could not be used on random mails, it was the marker for a "Changed" message,
if we would keep this names, things will fail for exising installations
as messages are dropped and a "Group image changed" message is shown instead.
