Test from both servers

zlib_test.php

<?php

function download ($url) {
	$headers = array(
		'User-Agent: Composer/source (Windows NT; 6.2; PHP 5.6.5)',
		'Accept-Encoding: gzip',
		'Connection: close',
	);

	$options = array(
		'http' => array(
			'follow_location' => 1,
			'max_redirects' => 20,
			'protocol_version' => 1.1,
			'request_fulluri' => true,
			'header' => $headers,
		),
	);

	if ('https' === parse_url($url, PHP_URL_SCHEME)) {
		$options['ssl']['SNI_enabled'] = true;
		if (PHP_VERSION_ID < 50600) {
			$options['ssl']['SNI_server_name'] = parse_url($url, PHP_URL_HOST);
		}
	}

	$options['http']['header'] = implode ("\n", $options['http']['header']);

	$ctx = stream_context_create($options);

	$stream = fopen($url, 'r', false, $ctx);

	$headersBeforeContents = stream_get_meta_data($stream);
	$content = stream_get_contents($stream);
	$headersAfterContents = stream_get_meta_data($stream);

	var_dump('start dump from ' . $url, $headersBeforeContents, $headersAfterContents, strlen($content), 'stop dump from ' . $url);
}

download('http://packagist.org/_p/symfony/finder%24a4bc31ef5d391952af0e7202b072f6d587ae89623277f89aefad0f9d49a9bc2c.json');
download('http://dump.robbast.nl/finder$a4bc31ef5d391952af0e7202b072f6d587ae89623277f89aefad0f9d49a9bc2c.json');