multiple file uploads to RPC timeout when using nginx proxy

#### Version information:
```
go-ipfs version: 0.4.18-aefc746
Repo version: 7
System version: amd64/linux
Golang version: go1.11.1
```

Client: `js-ipfs-api 26.1.2` running in node.

#### Type:
Bug

#### Description:
I'm having an issue uploading multiple files via the IPFS RPC when using an nginx reverse proxy. I am using the JS library `js-ipfs-api` in node. If I upload directly to port 5001 it works fine, but when I add the nginx proxy I get a timeout from the IPFS RPC. Ultimately my motivation is to use the nginx proxy for authentication to the IPFS RPC.

A few notes: I can upload multiple small files, but I start running into this issue when I have a couple files in the 5-7MB range. I am able to upload these files individually however.

I have created a repo to reproduce: https://github.com/pbostrom/ipfs_testcase
Install docker/docker-compose and run:
```
git clone https://github.com/pbostrom/ipfs_testcase.git
cd ipfs_testcase
sudo docker-compose up -d
sudo docker run -it --network ipfs_testcase_proxy-tier pbostrom/ipfs_testcase
```

This will twice attempt to upload 3 files: the first attempt directly to the RPC service on port 5001, the second attempt will go through the nginx reverse proxy. The second attempt should stall and nginx will eventually timeout. nginx logs:

```
2018/11/08 18:54:09 [error] 6#6: *1 upstream timed out (110: Connection timed out) while reading upstream, client: 172.18.0.4, server: , request: "POST /api/v0/add?recursive=true&stream-channels=true HTTP/1.1", upstream: "http://172.18.0.2:5001/api/v0/add?recursive=true&stream-channels=true", host: "nginx"
172.18.0.4 - - [08/Nov/2018:18:54:09 +0000] "POST /api/v0/add?recursive=true&stream-channels=true HTTP/1.1" 200 0 "-" "/node-ipfs-api/26.1.2/"
```

This may be related to #5168 but I do not see the error `http: invalid Read on closed Body` in any of the logs or responses.