httpd: add back immutable flag for static contents

The combination of a low max-age and immutable lets the web browser
cache local content for some time.

The web browser will still cache beyond that time, and so can issue
conditional requests (304 status.)

esp32/max80/httpd.c

@@ -655,7 +655,7 @@ static esp_err_t httpd_static_handler(httpd_req_t *req)
     len = snprintf(buffer, buffer_size-2,
 		   "HTTP/1.1 %s\r\n"
 		   "Date: %s\r\n"
-		   "Cache-Control: max-age=10\r\n"
+		   "Cache-Control: max-age=10, immutable\r\n"
 		   "ETag: %s\r\n",
 		   response,
 		   http_now(),