Fix case sensitivity of folder attribute parsing (\NoSelect, \NoInferiors) #469

[smajti1]

It should fix #469
Base on https://datatracker.ietf.org/doc/html/rfc3501#section-7.2.2 i also add check for \Noinferiors attribute

I have yet considered changing this function, to some thing like this:

protected function parseAttributes($attributes): void
{
	$normalized_attributes = array_map('strtolower', $attributes);

	$this->no_inferiors = in_array('\noinferiors', $normalized_attributes, true);
	$this->no_select = in_array('\noselect', $normalized_attributes, true);
	$this->marked = in_array('\marked', $normalized_attributes, true);
	$this->referral = in_array('\referral', $normalized_attributes, true);
	$this->has_children = in_array('\haschildren', $normalized_attributes, true);
}