Fix nested CollectionInputFilter not valid if count not specified

[gag237]

Problem using nested collections (CollectionInputFilter in another CollectionInputFilter).

inputFilter config example:

array(
   'type' => 'Zend\InputFilter\CollectionInputFilter',
   'name' => 'first_collection',
   'required' => true,
   'input_filter' => array(
       'type' => 'Zend\InputFilter\InputFilter',
       'second_collection' => array(
           'type' => 'Zend\InputFilter\CollectionInputFilter',
           'name' => 'second_collection',
           'required' => true,
           'input_filter' => array(
               'type' => 'Zend\InputFilter\InputFilter',
               'input' => array(
                   'name' => 'input',
               ),
           )
       )
   )
)

[weierophinney]

Cherry-picked to master for release with 2.3.2; merged to both master and develop.