CollectionInputFilter returns always valid for empty collections

[romankonz]

this pull request adds a required flag to an InputFilterCollection

if you have nested arrays, it's impossible to enfoce at least one entry.

$offers = array(
    array(
        'name' => 't-shirt',
        'prices' => array(
            // i need at least one price! 
                        // array('color' => 'black', 'price' => 149)
        ),
    ),
);

for example - this is very useful for mongodb documents.

[romankonz]

i think allowEmpty could be better as name