Add check for required fields to edd_purchase_form_validate_new_user()

Currently when you purchase a download as a new user, the form ignores the edd_purchase_form_required_fields() function which requires the first name by default. 

However, when you have an account ( logged in ) and purchase a download the form requires you to enter your first name, even though you didn't need to enter it when first registering. 

This patch will make the edd_purchase_form_validate_new_user() function behave the same as the edd_purchase_form_validate_logged_in_user() function.