2.3 only - Collection of minor fixes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | -
| Applies to    | 2.3
| Fixed tickets | -

Some minor fixes for 2.3 only.

The changes for the the Isbn/Issn/Iban validators are all about consistency.

The Isbn validator has no default option - currently two of them are declared to be the default.

Please comment if something is wrong - I'll do an update then.