Replace WPSC_Country::set() calls with constructor

Since WPSC_Country::set() is no longer supported, sending an array to
WPSC_Country::__construct() seems to create or update a listing as
needed.
Remove calls to WPSC_Country::save()

WPSC_Country::save() is deprecated, and no longer needed. The country
will be saved when the object is created.
Remove call to WPSC_Country::exists()

WPSC_Country::exists() is deprecated, but simply passing the info to
the WPSC_Country constructor should update and/or create as needed.
Merge pull request #1063 from jbeales/fix-deprecated-set-calls

Fix deprecated set calls