IteratorAssert should delay Iterator comsumption as much as possible

From Ansgar:

The following code will yield an AssertionError:

List<Integer> myList = Arrays.asList(1, 2, 3);
Iterator i = myList.iterator();
assertThat(i).isNotNull();
assertThat(i.hasNext()).isTrue();

Reason: assertThat() consumes the iterator. Should probably be done as late as possible, i. e. inside isNotEmpty().

This might lead (did in our project) to failing tests when switching from FEST-Assert 1.2 to 1.3 (as 1.2 did not have assertThat(Iterator<?>) and assertThat(Iterator<?>), so the Java compiler resolved assertThat to assertThat(Object) and the second assertion still succeeded).