    @Test
    public void asObservable() {

        Preference<String> preference = rxPreferences.getString("key1", "defValue");

        TestSubscriber<String> o = new TestSubscriber<>();
        Subscription subscription = preference.asObservable().subscribe(o);
        o.assertValues("defValue");

        rxPreferences.putString("key1", "value1");
        o.assertValues("defValue", "value1");

        rxPreferences.remove("key1");
        o.assertValues("defValue", "value1", "defValue");

        subscription.unsubscribe();
        rxPreferences.putString("key1", "foo");
        o.assertValues("defValue", "value1", "defValue");
    }