Issue description

APISIX supports rewriting of Host header while talking to the upstream nodes, either with the proxy-rewrite plugin or with "pass_host" and "upstream_host" attributes in Upstream configuration. However, they both rewrite host for the Upstream, not nodes - so for example if I have an upstream with nodes:

"nodes": {
    "host-1.com:80": 1,
    "host-2.com:443": 1
}, 

I can't set Host to "host-1.com" when the first node is picked by the balancer, and set Host to "host-2.com" when the second is picked. I have tried ngx.req.set_header() or set the upstream_host variable in balancer phase directly and they don't seem to work. Is it a constraint by nginx modules / openresty?

Environment

• apisix version (cmd: apisix version): 2.5
• OS (cmd: uname -a): Ubuntu 18.04
• OpenResty / Nginx version (cmd: nginx -V or openresty -V): 1.19.3.1
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API):
• apisix-dashboard version, if have:
• luarocks version, if the issue is about installation (cmd: luarocks --version):