Fix (Inflector::Methods#underscore): small regression

[Thornolf]

Summary

After this pull request was merged : dd5b00c, a tiny regression appeared on ActiveSupport::Inflector.underscore for some cases

Before when we were using the method underscore on Accountsv2N2Test it was returning this : accountsv2_n2_test but after the modifications, it was only returning this : accountsv2n2_test

To fix this we modified the regex from /([A-Z\d]+)(?=[A-Z][a-z])|([a-z\d])(?=[A-Z])/ to /([A-Z]+)(?=[A-Z][a-z])|([a-z\d])(?=[A-Z])/ and we added a little test case to it.

Please feel free to ask me any question about it.

Best regards,

Thornolf