Fix: tax: Only apply the tax rate once

My setup is the following. I am based in Germany and have a TaxZone for the VAT (19%), then I have a TaxZone for the rest of the European Union for the VAT (19%). I have this separation because if someone from the EU has a valid EUVAT Id I will not charge the VAT.

Somehow when placing an order for something like Italy, I ended up applying the tax rate twice. I think the TaxRate.match selector is wrong (and/or my ruby skills are weak).

I understand that this would require a testcase but I lack the time to write one and I am posting this pull request mostly as an indicator that this area of code still needs some work.