nginx: Relay the same Host: header that nginx saw.

Unilaterally adding the port can cause CSRF failures when the port is
a default port, and thus optional.  Switch to providing the exact
`Host` header that the original request contained.

(cherry picked from commit 5f783ed5ad)

puppet/zulip/files/nginx/zulip-include-common/proxy

@@ -2,7 +2,7 @@ proxy_http_version 1.1;
 # Clearing the Connection header is required for keepalives from the load balancer
 # http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive
 proxy_set_header Connection "";
-proxy_set_header Host $host:$server_port;
+proxy_set_header Host $http_host;
 proxy_set_header X-Forwarded-Proto $trusted_x_forwarded_proto;
 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 proxy_set_header X-Real-Ip $remote_addr;