allow serialize in json and xml alias

if `_serialize` is an array `array('items', 'success')` it's expected that the viewVars exist with these names. 

It's may not always be desired to use the name set in the controller as the outgoing variable name.

If you use `array('items' => 'data', 'went_ok' => 'succes');`  you can change the outgoing serialize value.

This may be an edge case feature, but for plugins like my Crud plugin it's very nice, and allow the system to much more flexible.