Add Fly.io header to precedence list

[mdalp]

Docs: https://fly.io/docs/networking/request-headers/#fly-client-ip

[iloveitaly]

@un33k seems like a good idea: what do you think?

[mdalp]

It is also true that the IP address is correctly computed from the X-Forwarded-For header anyway, but given there are few other cloud providers in the list, there is no harm to add one more.