Add default values to model

[waxim]

Models should be updated so default values can be assigned if an empty value is passed to the model.

$model = new Vouchers\Voucher\Model([
    'owner' => [
        'required'     => true,
        'immutable' => true,
        'validate'     => 'alpha|req|255',
        'default'      => 'default_owner'
    ],
    'claimed_on' => [
        'required' => true,
        'validate'  => 'date',
        'default'   => function() { return new DateTime(); }
    ]
]);