Unexpected behavior using to_json on a decorated object.

Hey guys,

First of all, thanks for the gem. We love it.

We had some unexpected behavior with draper and the `to_json` method. Basically, we were expecting it to behave like the following:

From a Rails 3.2.13 console.
```ruby
Task.include_root_in_json = true
 => true 
task.to_json(only: :id)
 => "{\"task\":{\"id\":1}}" 
Task.include_root_in_json = false
 => false 
task.to_json(only: :id)
 => "{\"id\":1}"
```

But with a decorated object, we get this unexpected behavior:
```ruby
Task.include_root_in_json = true
 => true 
task.decorate.to_json(only: :id)
 => "{\"task\":{\"id\":1}}" 
Task.include_root_in_json = false
 => false 
task.decorate.to_json(only: :id)
 => "{\"task\":{\"id\":1}}"
```

We have `include_root_in_json` set to false in an initializer like so:
```ruby
ActiveSupport.on_load(:active_record) do
  self.include_root_in_json = false
end
```

We worked around the issue by delegating `to_json` inside the decorator. I tried to write a good failing test to illustrate the unexpected behavior. I saw #101 which mentioned a similar situation - any thoughts on this?