Cannot subscribe to LiveData on the background thread

Hey Paulina, I like your lib, well done! :)
I'm getting `com.paulinasadowska.rxworkmanagerobservers.exceptions.LiveDataSubscribedOnWrongThreadException: cannot subscribe to LiveData on the background thread`.

I've tried adding to my subscription, but without success.
```
            .subscribeOn(AndroidSchedulers.mainThread())
            .observeOn(AndroidSchedulers.mainThread())
```