Calculating tax rate for logged-out customers

Creating this pull request as per [this support topic on the EDD forums](https://easydigitaldownloads.com/support/topic/eu-tax-rules-wrong-taxes-applied/#post-175095).

When calculating the tax rate for logged-out users, the function was checking for 'country' in the $_POST array. The field is named `billing_country` though, so it always defaulted to the store its operating country.