Documentation - specify that the ISO country code is required in upper case.

At the top of addressfield.inc, it would be helpful if it stated that the ISO country code is needed in upper case.

I found that from my testing, when the ISO code is in lower case, e.g. 'us', then the AddressField country field is not set correctly.

When $row->country is 'US', then the following code works and the AddressField country is set correctly.

    // The country should be passed in as the primary value.
    $this->addFieldMapping('field_event_location', 'country')
       ->arguments($arguments);