[2.3][Validator] Minor fix in IBAN validator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10481, #10489
| License       | MIT

Added more values to unit tests of IBAN validator to make clear it doesn't accept lower case letters.

> Permitted IBAN characters are the digits 0 to 9 and the 26 upper-case Latin alphabetic characters A to Z.
http://en.wikipedia.org/wiki/International_Bank_Account_Number

Also made little adjustment to code which meant to validate lowercase letters but actually was useless.