Don't allow forwarding of HPKP headers.

It could lead to a case where the site is effectively offline.

snippets/proxy-headers.conf

@@ -4,3 +4,5 @@ proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;
 proxy_set_header    Host $host;
 proxy_set_header    X-Real-IP $remote_addr;
 proxy_hide_header   Strict-Transport-Security;
+proxy_hide_header   Public-Key-Pins;
+proxy_hide_header   Public-Key-Pins-Report-Only;