Posting multipart/form-data

Hi, I'm trying to test a route that accepts multipart/form-data. The route works via postman, a node script, and in the browser, but I can't seem to get it to work with supertest (who referred me to the superagent docs).

Here's the code that's working in a simple node script:

```js
const form = new FormData()
form.append('entry', fs.createReadStream('./readme.md'))
await axios.post('http://localhost:3001/api/v0/files/add', form, {
headers: form.getHeaders()
})
// success!
```

I've tried numerous different strategies with forming a post via supertest:

```js
test('POST file', done => {
const form = new FormData()
form.append('entry', fs.createReadStream('./readme.md'))
request(app)
.post('/api/v0/files/add')
.type('form')
.attach('entry', form)
.set(form.getHeaders())
.expect(201, done)
})
```

I've also tried the above with different variations, like removing `.type` or `.set(headers)`, still with no luck.

The error seems to be throwing from the `busboy` lib, which is likely due to the multer lib:

```
Unhandled error. (Error: Part terminated early due to unexpected end of multipart data
at node_modules/busboy/node_modules/dicer/lib/Dicer.js:65:36
at processTicksAndRejections (internal/process/task_queues.js:75:11))Error [ERR_UNHANDLED_ERROR]: Unhandled error. (Error: Part terminated early due to unexpected end of multipart data
at node_modules/busboy/node_modules/dicer/lib/Dicer.js:65:36
at processTicksAndRejections (internal/process/task_queues.js:75:11))
at PartStream. (node_modules/busboy/lib/types/multipart.js:278:17)
at node_modules/busboy/node_modules/dicer/lib/Dicer.js:65:22
```

Here's the [issue](https://github.com/visionmedia/supertest/issues/602#issue-502243900) with supertest lib

any ideas what i'm doing wrong? Thanks for any help!