[5.0] Add sanitization to form requests

There are two points to this:

1. Sanitize the input before validation, so that we don't reject valid data.
2. Access the sanitized data in the controller, so that we don't have to sanitize twice.

---

Here's how you would use this:

The `FormRequest`:

```php
use Illuminate\Foundation\Http\FormRequest;
 
class UserFormRequest extends FormRequest {
 
	protected function rules()
	{
		return ['home_phone' => 'length:10'];
	}
 
	protected function sanitize(array $input, SomeSanitizer $sanitizer)
	{
		return $sanitizer->make($input, ['home_phone' => 'numeric'])->sanitize();
	}
 
}
```

The `Controller`:
 
```php
class UserController extends BaseController {
 
	public function store(UserFormRequest $request, User $user)
	{
		$user->fill($request->sanitized())->save();
 
		// or...
		$user->home_phone = $request->sanitized('home_phone');
		$user->save();
	}
 
}
```
`return array_get($input, $key, $default);` is enough, the check is done in `array_get`
https://github.com/laravel/framework/blob/4.2/src/Illuminate/Support/Arr.php#L212