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
=> #<Model id: nil, expired_at: nil>
[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...
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