bug: Upstream SNI should also be rewritten when the upstream host is rewritten

[tokers]

Issue description

Currently, when the upstream host is changed (by `proxy-rewrite plugin, or the Host rewrite policy in Upstream), the SNI passed to Upstream is not changed when the scheme is HTTPS. That might introduce some SSL handshaking problem, we may fix it.

Environment

• apisix version (cmd: apisix version):
• OS (cmd: uname -a):
• OpenResty / Nginx version (cmd: nginx -V or openresty -V):
• 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:

Minimal test code / Steps to reproduce the issue

What's the actual result? (including assertion message & call stack if applicable)

What's the expected result?

[membphis]

How to fix it? Are there any guides?

[tokers]

How to fix it? Are there any guides?

We should set the proxy_ssl_name directive with the upstream_host variable.

proxy_ssl_name $upstream_host;

Currently, the value of proxy_ssl_name is $host, which won't be changed even if the host to upstream was rewritten.

[membphis]

I think this issue can be a good first issue, right?

[tokers]

Yep.