In Rails 6, the new HostAuthorization middleware is enabled by
default (it's not enabled on other environments). It currently
allows localhost and IP addresses and we add
`Gitlab.config.gitlab.host` as additional allowed host.

When gitlab-shell makes a request via unix socket, it uses
`http://unix/<endpoint>` so the host will be `unix`. Since it's
not included in the list of allowed hosts, it gets blocked.

This adds `unix` host to the list of allowed hosts.