@Test
public void containsOnce_emitsFromInternalPreferences() {

    final String testKey = "Test key";
    when(mockSharedPreferences.contains(testKey)).thenReturn(true);
    final TestObserver<Boolean> subscription = rxPreferences.containsOnce(testKey)
            .test();
    subscriptions.add(subscription);
    subscription
            .assertComplete()
            .assertValueCount(1)
            .assertValue(true);
}