nginx proxy_redirect 无法配合upstream使用？

Question

upstream test_server {

server 127.0.0.1:8086 max_fails=3 fail_timeout=10s;

}
server {

listen 80;
server_name  xxx.example.com;
charset utf-8;
access_log off;

location / {
    proxy_redirect   http://test/   http://$host:$server_port;
    proxy_pass http://test_server;
}

}
浏览器URL直接就变成http://test_server了,如果使用 proxy_pass http://127.0.0.1:8086;就能正常访问