Zend\Http\Response::getBody() tries to decode gzip that has already been decoded by cURL

When performing a HTTP request to a gzipped resource via a cURL adapter that is configured to decode content, a following call to $httpResponse->getBody() fails because it looks at the still-set Content-Encoding header and tries to decode the content, which is at that point already decoded, again.

You can reproduce it like this:
```php
$httpClient = new Zend\Http\Client($urlWithGzipEncodedStuff,  [
    'adapter'      => 'Zend\Http\Client\Adapter\Curl',
    'curloptions'  => [
        // Instruct cURL to accept and decode any supported encoding.
        CURLOPT_ENCODING => '' 
    ]
]);
$httpResponse = $httpClient->send();

// BOOM: tries to decode what has already been decoded by  cURL
$content = $httpResponse->getBody();
```