childrenRecursiveSequence: fix NoSuchElementException being thrown when hasNext() returned true

[DarkEspresso]

This change addresses a problem caused by ViewChildrenRecursiveSequence's iterator next and hasNext both calling initItemIterator.

After hasNext returns true itemIterator is not null, therefore the condition
iterator == null || (!iterator.hasNext() && seqs.isNotEmpty())
in initItemIterator is equivalent to
!iterator.hasNext() && seqs.isNotEmpty()
which is false because iterator.hasNext() is true.

So initItemIterator in next() sets itemIterator to null, causing a NoSuchElementException to be thrown.

A test for the childrenSequence() and childrenRecursiveSequence() functions has been included.

[yanex]

Merged.
Thank you for the contribution!