useSTD3ASCIIRules does not work in v5.1.0

[yandrushchak]

Changes to useSTD3ASCIIRules handling in a recent commit broke the logic.
Reason - used regexp will only match 3-character strings, that start with letter, continue with digit, and end with hyphen, which can never happen. It should probably be changed to something like this: /^([a-z]|[0-9]|-)$/u

[domenic]

Thanks. It's unfortunate that these options don't all have sufficient test coverage.

A PR fixing this and adding such test coverage (in a new test file, I guess) would be welcome.

[yandrushchak]

@domenic
Created a PR to fix this, and added a new test suite to cover this option - #70.