debug=false validation added

If you call Guzzle client using `debug=false` (having curl disabled) you get printed debug information, since actually right now its just validating that the [key exists](https://github.com/mustela/RingPHP/blob/master/src/Client/StreamHandler.php#L200) but your are not checking the value.
Cool. Can you change this to:

```php
if ($value === false) {
    return;
}
```