Checkout: pass through currency type

Add passed currency type before falling back to default.
I was unfamiliar with the Nokogiri underscore paradigm and thought for a second that the original line had a typo in it as well.  So if there is a problem here, it seems it is because this convention looks like there is an unintentional underscore.  So even with parentheses it will look like an errant underscore to the uninitiated.  So I probably won't bother with an extra PR at this point.

However if I make any changes in this adapter in the future I wouldn't mind adding the parentheses to make the oder of operations more explicit.

Thanks for reviewing.