Data fetcher: Add Cache-Control header with max-age of a year.

Data URLs can't change, we want to assume they're fresh for
as long as we can.

content/fetchers/data.c

@@ -282,6 +282,12 @@ static void fetch_data_poll(lwc_string *scheme)
 						PRIsizet, c->datalen);
 			}
 
+			if (c->aborted == false) {
+				/* Set max-age to 1 year. */
+				fetch_data_send_header(c, "Cache-Control: "
+						"max-age=31536000");
+			}
+
 			if (c->aborted == false) {
 				msg.type = FETCH_DATA;
 				msg.data.header_or_data.buf =