Fixed bug about obtaining args in Model_Soft::__callStatic

The following code fails because using `array_pop` to obtain arg. I think using `array_shift` is correct in this case.

```php
class Model_Something extends Model_Soft { }

// error
Model_Soft::find_deleted('all', array(...));
```

Signed-off-by: Hiroyuki Anai <pirosikick@gmail.com>