Node Download Progress Never Fires

I'm trying to use the `.on('progress', cb)` method for a download in my Node application, but the callback function is never called. For uploads, `.on('progress', cb)` works just fine.

Relevant versions:
- node: 12.14.1
- superagent: 6.1.0
- typescript: 4.0.5
- ts-node: 9.0.0

Download example (does not work):
```ts
agent()
.get(downloadUrl)
.maxResponseSize(this.maxDownloadSize ?? 200_000_000)
.on('progress', ev => {
console.log('Progress: ', ev.loaded);
})
.then(r => {
console.log("Response body: ', r.body);
});
```

Upload example (works):
```ts
agent()
.post(uploadUrl)
.attach('file1', bufferToUpload, {filename: 'thing1.zip', contentType: 'application/zip'})
.on('progress', ev => {
console.log('Progress: ', ev.loaded);
})
.then(r => {
console.log("Response body: ', r.body);
});
```