I use recreate_request in balancer_by_lua_block phase, but it never work.

I want to change request header in balancer_by_lua_block phase, because i need to pass host header to next upstream node. I use recreate_request function to change request header information but it doesn't work.

my test example show as follow:

`upstream apisix_backend {

    server 0.0.0.1;
    balancer_by_lua_block {
        local balancer = require "ngx.balancer"
        balancer.recreate_request()
        ngx.var.domain_host="www.baidu.com"
    }

    keepalive 320;
}

`

`
location / {
set $upstream_host '';
proxy_set_header Host $upstream_host;

        content_by_lua_block {
               ngx.say("1");
       }

}
`

Start by reproducing the configuration in the issue, focusing on ngx.balancer.recreate_request in the balancer_by_lua_block phase and the interaction with ngx.var.domain_host and proxy_set_header Host. Check the ngx.balancer API behavior and related balancer-phase handling. Done means establishing whether the header change is supported there and fixing or clearly documenting the observed behavior.