Fixes incorrect order.item_total when line items have multiple currencie...

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Given an order having line items in multiple currencies, order.total will simply add the line item prices and claim the total is in USD, even if a line item price is in another currency.

Undoubtedly, this PR will spark some discussion regarding the right way to handle this.  I have proposed one solution: force every line item to have the current order currency.

I'm not happy about where I put this code.  I wanted to do it in a before_validation hook on 'order', but the order_updater bypasses all validations.

So I have provided a test that illustrates the issue and I'd love to have some discussion on the proper way to handle the scenario.  

thanks to @jdutil for some input.