Move Obj stringify step to _prepare

[hueniverse]

The object response isn't consistent with the rest of the response types due to performing JSON.stringify at construction time instead of during the _prepare stage. This requires supporting the update() method to regenerate the internal payload buffer.

This change will remove the update() method and will delay conversion until right before onPreResponse is called. It will also require creating a new response object in order to make changes to the internal raw data (which by itself can be reused).