Return same instance instead of wrapping Iterator results when possible

[danieldietrich]

1. Remove the method useIsEqualToInsteadOfIsSameAs() from unit tests and test for isSameAs()
2. fix broken unit tests

Example: take(0)

// AbstractTraversableTest
@Test
public void shouldTakeNoneOnNil() {
    if (useIsEqualToInsteadOfIsSameAs()) {
        assertThat(empty().take(1)).isEqualTo(empty());
    } else {
        assertThat(empty().take(1)).isSameAs(empty());
    }
}

// PriorityQueue (current)
@Override
public PriorityQueue<T> take(int n) {
    return ofAll(comparator, iterator().take(n));
}

// PriorityQueue (time and mem saver)
@Override
public PriorityQueue<T> take(int n) {
    if (n <= 0) {
        return this;
    } else if (n >= size()) {
        return empty(comparator());
    } else {
        return ofAll(comparator, iterator().take(n));
    }
}

This was only an example. We have many similar occurrences.