Fix http 404 when BASE_PATH is assigned

[qiangyt]

Below is the nginx reverse proxy configuration which I used to test.

location /admin { proxy_pass http://127.0.0.1:5000/admin; proxy_http_version 1.1; proxy_set_header Host $server_name; proxy_buffering off; proxy_set_header X-Real-IP $remote_addr; proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for; proxy_set_header X-Forwarded-Proto $http_x_forwarded_proto;

    auth_basic "Administrator's Area";
    auth_basic_user_file /etc/nginx/htpasswd;
}

[iFargle]

(reverted by mistake, his changes are in the code. Again, thanks for contributing!)