Make the tests compatible with Rails 4  

- `Model.exists?` returns falsy in Rails 4

> [Model.exists?] Returns truthy if a record exists in the table that matches the +id+ or conditions given, or falsy otherwise.

Please see the discussion https://github.com/rails/rails/issues/10540 and the commit https://github.com/rails/rails/commit/aec0e68

- The `where_values_hash` no longer an instance of HashWithIndifferentAccess. So we have to use a string to access it.