Fix form model binding

Form model binding now works properly with a mix of nested arrays, objects and Eloquent collections.

This does make the assumption that people index their relationship inputs using the related model IDs, for example:

```php
<?= Form::model($model) ?>
<?php foreach($model->relation as $related): ?>
  <?= Form::text("related[{$related->id}][name]") ?>
<?php endforeach ?>
```
Here's what I meant:
```php
if ($data instanceof EloquentCollection)
{
	$data = $data->find($key);
}
elseif (is_array($data) or $data instanceof \ArrayAccess)
{
	$data = isset($data[$key]) ? $data[$key] : null;
}
```