nginx module: fix applying recommended proxy headers

Previously, if proxy_set_header would be used in an extraConfig of
a location, the headers defined in the http block by
recommendedProxySettings would be cleared. As this is not the intended
behaviour, these settings are now included from a separate file if
needed.

nixos/modules/services/web-servers/nginx/location-options.nix

@@ -14,7 +14,8 @@ with lib;
       default = null;
       example = "http://www.example.org/";
       description = ''
-        Adds proxy_pass directive.
+        Adds proxy_pass directive and sets recommended proxy headers if
+        recommendedProxySettings is enabled.
       '';
     };