[PATCH] Add custom root option to AR JSON Serializer

In short: I can do following:
user.to_xml(:root => "app_user" )

But not following:
user.to_json(:root => "app_user")

JSON serializer for AR doesn't allow us to specify "root" option unlike XML serializer. JSON serializer always uses model name as the root if class attribute include_root_in_json is set to true.

I've attached a patch for master and 2.3.