Region dropdown missing sub-array element (Bug Fix)

Working with a fellow developer - we determined that in checkout data that the Billing Region Dropdown didn't pass both the region ID as well as the Country ISO (as a two-element array for billingcountry)
The bug is that valid region/country pairs failed validation. I think this addresses some errors reported as issues but haven't specifically confirmed. (But the handling of billing region/country now matches shipping region/country)