[WIP] Fix purge syntax for Varnish 4.0.1

This fixes #97.
Okay, the issue is probably PHP-related, perhaps `curl_multi`, which is used by Guzzle 3. The following gives the expected result:

```php
        $context = stream_context_create($opts = array('http'=>array('method'=>"GET", 'header'=>"Accept:application/json\r\nUser-Agent: ding-dong\r\nnHost:localhost:6181")));
        echo file_get_contents('http://localhost:6181/negotation.php', false, $context);
        print_r($http_response_header);

        $context = stream_context_create($opts = array('http'=>array('method'=>"GET", 'header'=>"Accept:application/json\r\nUser-Agent: ding-dong\r\nnHost:localhost:6181")));
        echo file_get_contents('http://localhost:6181/negotation.php', false, $context);
        print_r($http_response_header);

        $context = stream_context_create($opts = array('http'=>array('method'=>"GET", 'header'=>"Accept:text/html\r\nUser-Agent: ding-dong\r\nHost:localhost:6181")));
        echo file_get_contents('http://localhost:6181/negotation.php', false, $context);
        print_r($http_response_header);
```