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.
For consistency with other tests that use example public and private
keys, replace use of `concat!` macro with `include_str!`. This way, key
data embedded into source with single line of code.
Additionally, this change reduces duplication: now test public key is
stored in only one place, and used in two instead of copy-paste of very
long line.
