Fixing issue 168 (missed DK code)

Added DK to iso alpha 2 country code list. Now it is iso compliant
(https://www.iso.org/obp/ui/#search/code). Since this list has now 249
items as per iso.org
Merge pull request #170 from jhmilan/issue-168

GH-168: Adds "DK" to country code validation rule.