nginx: use application/octet-stream content-type for WKD routes

The format of keys returned on WKD routes is binary, however
`application/pgp-keys` is specified to contain keys in ASCII-armored
format. The WKD spec says the returned content-type SHOULD be
`application/octet-stream`, too.

references:
https://www.ietf.org/archive/id/draft-koch-openpgp-webkey-service-11.txt
https://tools.ietf.org/html/rfc3156#section-7

hagrid-routes.conf

@@ -110,7 +110,7 @@ location /.well-known/openpgpkey {
         error_page 429 /errors-static/429-rate-limit-vks-email.htm;
 
         error_page 404 /errors-static/404-wkd.htm;
-        default_type application/pgp-keys;
+        default_type application/octet-stream;
         add_header Content-Disposition 'attachment; filename="$2$3$4.asc"';
         add_header 'Access-Control-Allow-Origin' '*' always;
         add_header 'Cache-Control' 'no-cache' always;