Settings.php conf overrides are incorrect

Currently we find this in settings.php

$conf['system.performance']['css.gzip'] = FALSE;
$conf['system.performance']['js.gzip'] = FALSE;

This is incorrect as everything behind system.performance is an array so we have to access it as an array
Following code DOES work.

$conf['system.performance']['css']['gzip'] = FALSE;
$conf['system.performance']['js']['gzip'] = FALSE;

System.performance is a KEY js.css are two ARRAY ELEMENTS. We can't combine array elements with a '.'

Attached patch fixes this misunderstanding. I tested this with css/js aggregation.