Make default tax not apply to non-matching zones.

Tax Rates can be flagged as a default tax rate, and it will apply to
all orders if there is no information to base a tax rate off of.

However, once we have enough information to determine the appropriate
tax rate for a user (i.e. a tax_address, which is the billing address
or shipping address) we should use that to determine the tax rate
which will apply.

Previously, the default tax rate would match for all orders regardless
of the zone specified in the order.

Fixes #2335.