Pull request: filtering: fix letter case in cname matching

Updates #3335.

Squashed commit of the following:

commit ff55c112417199e4b04098a32c5f4805b59356f9
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Jul 12 12:38:46 2021 +0300

    filtering: fix letter case in cname matching

internal/filtering/filtering.go

@@ -403,6 +403,8 @@ func (d *DNSFilter) CheckHostRules(host string, qtype uint16, setts *Settings) (
 		return Result{}, nil
 	}
 
+	host = strings.ToLower(host)
+
 	return d.matchHost(host, qtype, setts)
 }