Prevent selected from being included twice in the HTML for country selector helper

[damonmorgan]

prevent selected from being included twice in the HTML which causes
some browsers to select the second selected option (not priority)
which makes it harder to select an alternative priority country

fix taken from https://github.com/jamesds/country-select/blob/master/lib/country-select.rb

[hexorx]

Thanks again! That is a better way to handle it.