Fix parsing of raw ipv6 address in host header

[luzrain]

Fixed problem:
If you expose ipv6 and try to get access the server using a raw ipv6 address, you will get a 400 error because regexp could not parse ipv6 properly.

Test code to reproduce
Try to get access to http://127.0.0.1:8085 and http://[fe80::215:5dff:fe67:1d75]:8085 (change ipv6 to your local)

use Amp\Http\Server\DefaultErrorHandler;
use Amp\Http\Server\Request;
use Amp\Http\Server\RequestHandler;
use Amp\Http\Server\Response;
use Amp\Http\Server\SocketHttpServer;
use Psr\Log\NullLogger;

$logger = new NullLogger();
$errorHandler = new DefaultErrorHandler();
$requestHandler = new class implements RequestHandler {
    public function handleRequest(Request $request) : Response
    {
        return new Response(body: 'Hello, world!');
    }
};

$server = SocketHttpServer::createForDirectAccess($logger);
$server->expose('0.0.0.0:8085');
$server->expose('[::]:8085');
$server->start($requestHandler, $errorHandler);

// Serve requests until SIGINT or SIGTERM is received by the process.
Amp\trapSignal([SIGINT, SIGTERM]);

$server->stop();