client: don't advertise keep-alive in uh_header_error()

The function `uh_header_error()` will tear down the connection after
sending the HTTP reply message, so update the `connection_close` flag
accordingly to avoid incorrectly emitting a `Connection: keep-alive`
header.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

Author: jow-

client.c

@@ -168,6 +168,9 @@ uh_client_error(struct client *cl, int code, const char *summary, const char *fm
 
 static void uh_header_error(struct client *cl, int code, const char *summary)
 {
+	/* Signal closure to emit the correct connection headers */
+	cl->request.connection_close = true;
+
 	uh_client_error(cl, code, summary, NULL);
 	uh_connection_close(cl);
 }