upload File: update operation in mode field is not working

Hi, Steve.

I also encountered exactly same problem.

I'm developing iOS app and switching from dropbox-ios-sdk to SwiftyDropbox(v1 -> v2).

And almost features works fine, but I can't resolve this problem with upload feature.

I check upload feature with SwiftyDropbox and curl command.

And when I try to upload some files with curl command, it won't work as I expected.
So I suspect upload api.

```
curl -X POST https://content.dropboxapi.com/2/files/upload \
    --header "Authorization: Bearer ***********************************************************************************************” \
    --header "Dropbox-API-Arg: {\"path\": \”/sample.jpg\",\"mode\": \”add\”,\”autorename\": true,\"mute\": false}" \
    --header "Content-Type: application/octet-stream" \
    --data-binary @sample.jpg
```

{"name": "sample.jpg", "path_lower": "/sample.jpg", "path_display": "/sample.jpg", "id": "id:********************", "client_modified": "2016-08-27T13:55:11Z", "server_modified": "2016-08-27T13:55:11Z", "rev": "135cc0934ed6f", "size": 527359}

I expect that file name which is included within response changes like below when I call api several times, but api returns above result when I call api several times.
Is my expectation is wrong?

sample.jpg
sample1.jpg
sample2.jpg
sample3.jpg
...