thoughtbot/shoulda-matchers — pull request #510 by nilcolor (2014-04-25 14:50:06+00:00)
Title: Within ValidateAbsenceOfMatcher, cast @attribute to String.

ActiveRecord.columns_hash returns Hash with String keys.
This is why `it { expect(offer).to validate_absence_of(:expired_at) }` is not working - setting 'an arbitrary value' to a time field is not working.

```
[1] pry(main)> rec = Model.new
=> #
[2] pry(main)> rec.expired_at
=> nil
[3] pry(main)> rec.expired_at = 'hi! this is me'
=> "hi! this is me"
[4] pry(main)> rec.expired_at
=> nil
```
this is why `validate_absence_of` can not find error...