GROOVY-6122 NodeChildren/FilteredNodeChildren

Fixes a problem with the way that FilteredNodeChildren inherited the childNodes Iterator. Now these GPathExpression do no longer fail:

    assert records.car.findAll({ true } ).country == 'AustraliaIsle of ManFrance'
    assert records.car.findAll({ it.@make =~ /e/ }).record.@type == 'speedsize'
    assert records.car.findAll({ true }).findAll({ it.@make =~ /ug/ }).country == 'France'

Also removing the overridden implementation of iterator(). FilteredNodeChildren can rely on the parent implementation, if that is simplyfied by just wrapping its own nodeIterator().

All other asserts from the XmlSluper doc work fine as well, including XML modification.