Description:
------------
When the curl handle is reused, curl_getinfo() contains the previous 
download_content_length value if the current url header contains no 
Content-Length: header.

Reproduce code:
---------------
$url_list = array('http://example.com', 'http://php.net');
$ch = curl_init();
curl_setopt($ch, CURLOPT_NOBODY, true);
curl_setopt($ch, CURLOPT_HEADER, true);
foreach ($url_list as $each) {
    curl_setopt($ch, CURLOPT_URL, $each);
    curl_exec($ch);
    echo curl_getinfo($ch, CURLINFO_CONTENT_LENGTH_DOWNLOAD) . "\n";
}
curl_close($ch);


Expected result:
----------------
"-1" as the last value, since the response from the second url 
(http://php.net) contains no Content-Length: line.

Actual result:
--------------
HTTP/1.1 200 OK
...(header lines)
Content-Length: 438

438

HTTP/1.1 200 OK
...(header lines

438