Custom service desk handler ignores additional headers

In the Gitlab::Email::Receiver we try to find key from multiple places in the email header.

That does not happen when a custom address is used for service desk. In that case, ServiceDeskReceiver is used and only to is being checked..

We should check also other keys.

That is problematic, when a cc-ed user responds and tries to comment an issue as implemented in !71749 (merged).