@Test
public void download() throws Exception {

    final String fileId = "fileId";
    final java.io.File file = new java.io.File("/");
    when(driveDataStreams.download(fileId, file)).thenReturn(Single.just(Optional.of(file)));
    final TestObserver<Optional<java.io.File>> testObserver = driveStreamsManager.download(fileId, file).test();
    testObserver.assertValue(Optional.of(file));
    testObserver.assertComplete();
    testObserver.assertNoErrors();
}