Checkout: pass through currency type

Add passed currency type before falling back to default.
No problem! Looking through [some docs on this](http://ruby-doc.org/gems/docs/n/nokogiri-fitzsimmons-1.5.5.3/Nokogiri/XML/Builder.html#label-Special+Tags), it looks like unless `bill_amount` was a reserved word the original trailing underscore isn't needed either. I think it might be ok to remove the underscores from most of these calls (if not all of them) the next time this adapter gets updated.

Thanks for the explanation!