truthy default_configs can't be overridden

Given 
```ruby
require 'pp'
require 'kitchen'

module Kitchen

  module Driver

    class Truthy < Kitchen::Driver::SSHBase
      default_config :truthy, false

      def create(state)
        puts "state"
        pp state
        puts "---\nconfig"
        pp config
      end
    end
  end
end
```
 and
```yaml
---
driver_plugin: truthy

platforms:
- name: ubuntu-12.04
  driver_config:
    truthy: true

suites:
- name: default
  run_list: []
  attributes: {}
```
I get 
```
-----> Starting Kitchen (v1.0.0.dev)
-----> Creating <default-ubuntu-1204>
       state
{:last_action=>"create"}
       ---
config
{:truthy=>true,
 :kitchen_root=>"/Users/tmay/work/truthy-test",
 :provisioner=>"chef_solo",
 :sudo=>true,
 :port=>22}
       Finished creating <default-ubuntu-1204> (0m0.00s).
-----> Kitchen is finished. (0m0.01s)
```
as expected. 
However, if I change the driver to have
```ruby
default_config :truthy, true
```
and kitchen.yml to have
```
truthy: false
```
I still get
```
-----> Starting Kitchen (v1.0.0.dev)
-----> Creating <default-ubuntu-1204>
       state
{:last_action=>"create"}
       ---
config
{:truthy=>true,
 :kitchen_root=>"/Users/tmay/work/truthy-test",
 :provisioner=>"chef_solo",
 :sudo=>true,
 :port=>22}
       Finished creating <default-ubuntu-1204> (0m0.00s).
-----> Kitchen is finished. (0m0.01s)
```