FIX: repeat the proxy_set_header lines in the @discourse block

My previous understanding of this was incomplete: `proxy_set_header` only has
an effect when proxy_pass is *directly* used.

In our nginx configuration file, we have two paths to get from the `location /` main block to the upstream:

1: `location /` → `proxy_pass http://discourse` → `upstream discourse`
2: `location /` → `try_files @discourse` → `proxy_pass http://discourse` → `upstream discourse`

In the first case, the `proxy_set_header` directives from the `location /` block
(or one of its sub-blocks) takes effect and the headers are set as expected.

In the second case, the `proxy_set_header` directives from the `location /`
block are *not used* since `proxy_pass` was not used from that location.

Only the `proxy_set_header` directives from the `location @discourse` block are
considered since that is the configuration block that calls `proxy_pass`

config/nginx.sample.conf

@@ -261,6 +261,17 @@ server {
   }
 
   location @discourse {
+    # fallback to discourse
+    #
+    # repeat the default proxy_set_header lines from above because we didn't
+    # get here via a proxy_pass statement
+    proxy_set_header Host $http_host;
+    proxy_set_header X-Real-IP $remote_addr;
+    proxy_set_header X-Request-Start "t=${msec}";
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Proto $thescheme;
+    proxy_set_header X-Sendfile-Type "";
+    proxy_set_header X-Accel-Mapping "";
     proxy_pass http://discourse;
   }
 }