[Minor] Add milter flag when using milter protocol

src/libserver/milter.c

@@ -1326,6 +1326,7 @@ rspamd_milter_to_http (struct rspamd_milter_session *session)
 	}
 
 	rspamd_milter_macro_http (session, msg);
+	rspamd_http_message_add_header (msg, MILTER_HEADER, "Yes");
 
 	return msg;
 }