unzip does not work when using pipe

simple fix, there's a lot of stuff going on in utils.unzip so I just skipped that and made it work.

But, this is total crap because it unzips the thing twice because it still runs https://github.com/timoxley/superagent/blob/e36814a3e5a9b95fc68fb7a76bf2feb79a5772ee/lib/node/index.js#L679-L682
```js
// zlib support
if (/^(deflate|gzip)$/.test(res.headers['content-encoding'])) {
  utils.unzip(req, res);
}
```

Which, I guess, isn't any more inefficient than it the current code, since using `.pipe` also unzips the data but it just does nothing with it, haha great.

This makes it work, but yeah sucks, and overall that area's code could do with straightening out.