sanitize 2nd parameter can be a whitelist fields

The method now returns a new array with only the elements allowed by $fields param.
Useful for ORM or active record: the sanitized array must contain only keys that match table columns.

With this edit you can "whitelist" the submitted data passing an array of allowed keys/columns as 2nd parameter sanitize() method. All the array keys not included will be ignored and will be not in the sanitized array.

Before I explained here in an issue: 
https://github.com/Wixel/GUMP/issues/48