When a priority region is selected, don't select it twice in the markup

If a priority region is selected on the `country_select`, it is selected again in the main list. This causes some browsers (notably, the latest version of Chrome) to select the second occurrence of this region, which makes it difficult to select an alternative priority region.

Added a spec to cover this, and ensured the spec suite passes.

See also [this pull request](https://github.com/jamesds/country-select/pull/3).

P.S. Thanks a lot for this gem, and for Carmen. The codebase and test suite were a real pleasure to navigate, and modify. Great to see that every once in a while!