Fix tax calculator with non-VAT taxes.

This is an issue with Spree line items and tax rate calculators.

The method used to subtract tax from a VAT did not play nicely
with non value added taxes.

Previously the test would show that 5% of $30 is $1.43 instead of $1.50.

$30.00 - $1.43 = $28.57
$28.57 * 1.05 =~ $30

So the code makes sense for VAT's only.  It should now work for both types of tax.