accepts_value_for(:do_cleanup, false) does not setup value to be false

Hello Ben,

is it expected behavior? :)

```ruby
babushka@8311af7 | ruby-1.9.3p448 | rspec-2.14.1
  1) accepts_*_for accepts_list_for should return the correct boolean default when no value is stored
     Failure/Error: subject.do_cleanup.should == [false]
       expected: [false]
            got: nil (using ==)
     # ./spec/babushka/accepts_for_spec.rb:85:in `block (3 levels) in <top (required)>'

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1098 examples, 1 failure
```