Add port to to the header in Nginx config (#5101)

* Add port to to the header in Nginx config

Using `$host` only for the header in Nginx config causes that Kestrel knows only that its host name is *hostname.com* without any port. That will cause problems in redirecting (e.g. to login page). I added the `$server_port` as well. So the hostname will become *hostname.com:8080* for example. That solves any related redirecting problems.

* Using $http_host in nginx config

`$http_host` is more accurate that `$host` because if includes the port number. using `$host` will cause problems when running kestrel behind reverse proxy on ports other than the standard ports (80 and 443).

aspnetcore/publishing/linuxproduction.md

@@ -116,7 +116,7 @@ server {
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection keep-alive;
-        proxy_set_header Host $host;
+        proxy_set_header Host $http_host;
         proxy_cache_bypass $http_upgrade;
     }
 }