The progress=true option breaks upload via an Nginx reverse proxy

#### Version

I'm using a docker container: `ipfs/go-ipfs:v0.4.20``
```
/ # ipfs version --all
go-ipfs version: 0.4.20-
Repo version: 7
System version: amd64/linux
Golang version: go1.12.4
```

#### Description

When using the `progress=true` flag with the `/api/v0/add` API path the upload just stops midway for files larget than a few MB(the larger the more likely to stop).

#### Steps to reproduce

0. Configure an Nginx reverse proxy for port `5001`:
```nginx
server {
listen 8888;

proxy_set_header Connection "";
proxy_http_version 1.1;

proxy_connect_timeout 600s;
proxy_send_timeout 600s;
proxy_read_timeout 600s;

client_max_body_size 50m;

location /api/v0/add {
proxy_pass http://localhost:5001;
}
}
```

1. Create a 4MB file:
```
dd if=/dev/urandom of=file.txt count=4 bs=1M
```

2. Attempt to upload it with `progress=true` enabled:
```
curl -v -F 'file=@file.txt' 'http://localhost:8888/api/v0/add?progress=true'
...(omitted)...
{"Name":"file.txt","Bytes":262144}
...(omitted)..,

{"Name":"file.txt","Hash":"Qmav1M4BBDmN2pYAy5GoetdHqV7MJX489NnKTUPi9BUT3n","Size":"4195305"}
* Connection #0 to host localhost left inta
```

3. Create another file that is 10MB:
```
dd if=/dev/urandom of=file.txt count=10 bs=1M
```

4. Try to upload again:
```
curl -v -F 'file=@file.txt' 'http://localhost:8888/api/v0/add?progress=true'
...(omitted)...
< HTTP/1.1 100 Continue
< HTTP/1.1 200 OK
...(omitted)...
{"Name":"file.txt","Bytes":262144}
...(omitted)...
{"Name":"file.txt","Bytes":7077888}
```
And then it just stops, indefinitely.

If the `progress=true` option is omitted everything works just fine.

#### Details

Naturally the upload works just fine without the proxy. I've tried watching the traffic with `tcpdump` to figure out who stops responding and it seems to be the IPFS service. The nginx proxy passes a chunk of data, and then IPFS doesn't respond, and everything stops.