Add missing specs for handled HTTP status codes

Add missing specs for handled HTTP status codes

https://gitlab.com/gitlab-org/gitlab-shell/blob/master/lib/gitlab_net.rb#L39

resp = post(url, params)

case resp.code.to_s
  when HTTP_SUCCESS, HTTP_UNAUTHORIZED, HTTP_NOT_FOUND
    GitAccessStatus.create_from_json(resp.body)
...