Sanitize latinamerican spanish

`#sanitize_available_locales` did not work when called on the Latin America Spanish "es-419" locale:
```rb
HttpAcceptLanguage::Parser.new("").sanitize_available_locales(["es-419"])
# => ["es"]
```
With this patch, the method will return `["es-419"]`.

The `start_with?("x")` may be over-simplified, but so are the specs ;-)

cc/ @pinkvelociraptor @grosser