File upload never ends?

On MacOSX with Docker and the following versions of relevant libraries:
```
channels==2.0.2
Django==2.0.2
djangorestframework==3.7.7
daphne==2.0.3
asgiref==2.1.6
```

(I also tried a `pip install -U -r requirements.txt` to try and catch any new versions, believe I did that properly and didn't see any available upgrades)

I found this thread with a similar problem #753 and the problem seems to have been solved, but I experience a successful POST + upload with no response from the server (it just perpetually hangs).

```bash
django_1 | [2018/02/22 04:54:16] HTTP GET /api/submissions/ 200 [0.11, 172.19.0.1:57416]
django_1 | 2018-02-22 04:54:26,527 - WARNING - server - Application instance

wait_for=._call_check_cancel() at /usr/local/lib/python3.6/asyncio/futures.py:403, ()]>>
for connection took too long to shut down and was killed.
```

What is so confusing is that everything seems to work fine, but I just don't get a response. The object is created, the file is successfully uploaded..

A little unsure how best to debug this, not very familiar with ASGI/async/channels. Would appreciate a push in the right direction and I apologize if this issue is too vague

Everything else about channels seems to be working swell, don't mean to sound ungrateful! Thanks so much for releasing this and all the hard work :)