In Zend\Http\Client::prepareHeaders (https://github.com/zendframework/zf2/…):

        if (! isset($this->headers['accept-encoding'])) {
            if (function_exists('gzinflate')) {
                $headers['Accept-encoding'] = 'gzip, deflate';
            } else {
                $headers['Accept-encoding'] = 'identity';
            }
        }

The "$this->headers" property is not used anywhere else, perhaps the "if" line should be something like that, according to the surrounding code:

if (!$this->getRequest()->getHeaders()->has('Accept-Encoding')) {
...
}

Now the condition is always true and there is no other option to disable gzip in the HTTP response than removing the gzip PHP extension.