Made Response instance the only valid return type for controller action.
Update requestAction() test methods.

These methods instead of returning arrays now set the reponse body
with serialized data.
Changed serialize/unserialize to json encode/decode.

This better represents potential real world usage.
Merge pull request #3138 from ADmad/3.0-controller-action

3.0 - WIP - Made Response instance the only valid return type for controller action.