Set transfer encoding for multipart/related to chunked by default

Fixes https://github.com/request/request/issues/1271

Also added a test for `GET` request.

Basically the test for this feature is

```js
if (chunked) {
  t.ok(req.headers['transfer-encoding'] === 'chunked')
  t.notOk(req.headers['content-length'])
}
```

~~Inside the `end` event of the request there is no body when `GET` request is being used.~~