Add timestamptz to the dummy value case statement

[callahat]

Ran into an exception when upgrading to Rails 7.0.4.2 and a column of type timestamptz.

     Failure/Error:
       is_expected.to validate_uniqueness_of(:amount).
         scoped_to(:column1, :column2, :column3, :date_time_column).
         with_message('must be unique')
     
     NoMethodError:
       undefined method `getutc' for nil:NilClass

After tracing, 'dummy value' was observed as being sent into this function, when then resulted in time being nil:
https://github.com/rails/rails/blob/v7.0.4.2/activerecord/lib/active_record/connection_adapters/postgresql/oid/timestamp_with_time_zone.rb#L122

[mcmire]

Hi @callahat, thanks for the PR. Would you mind adding tests for this so that the bug doesn't reappear later accidentally? You can see an example of existing tests here:

shoulda-matchers/spec/unit/shoulda/matchers/active_record/validate_uniqueness_of_matcher_spec.rb

Line 832 in 195153c

context 'when one of the scoped attributes is a time column (using Time)' do

Thanks!

[callahat]

@mcmire Tests added for timestamptz as well as timestamp (the later already existed, but didn't have a similar test to datetime, and all three are in the same when clause)

[mcmire]

@callahat Great, thanks so much!