SequentialAccessSparseVector inerateNonZeros is incorrect.

In SequentialAccessSparseVector a bug is noted. When Cuonting Non-Zero elements NonDefaultIterator can, under certain circumstances give an incorrect iterator of size different from the actual non-zeroCounts.

 @Override
  public Iterator<Element> iterateNonZero() {

    // TODO: this is a bug, since nonDefaultIterator doesn't hold to non-zero contract.
    return new NonDefaultIterator();
  }