Added $attributes check in self::DELETE

Trying to use `PodioItem::delete($itemID, array(), array("silent" => "true")` results in
````
Fatal error: Uncaught PodioBadRequestError: "Invalid value "1?" (string): must be {'1', 'on', 'true', 'y', 'yes', True, '0', 'false', 'n', 'no', 'off', False}”
Request URL: http://api.podio.com/item/123456789?silent=1?
Request Body: null
````
due to the extra question mark being inserted. This solves the problem.