Implement got as WritableStream on POST and PUT

This will allow write to Stream, that returned from got, if method is `POST` or `PUT`.

Would it be useful to add some shortcuts?

```js
stream.pipe(got('http://devnull-as-a-service.com/', { method: 'POST' }))
// vs
stream.pipe(got.post('http://devnull-as-a-service.com/'))
```
Done.