Only check once whether we're in a non-default non-order zone

[mamhoff]

When calling Spree::TaxRate#adjust, Spree::TaxRate#compute_amount will
come up with negative amount only if we're processing a refund. This commit
checks for the amount of the reimbursement to be negative instead of running
a rather expensive call to Spree::Zone.contains.

[cbrunsdon]

👍, its slightly unclear why that check is happening to anyone not deeply familiar with this chunk of code, but as we are (lols, "we") working on removing it entirely I'm cool with it

[jhawthorn]

Took me a minute to fully understand. The compute_amount method is doing the same check, and setting the amount to negative only if included_in_price && !default_zone_or_zone_match. So if included_in_price && amount > 0, we know that default_zone_or_zone_match is truthy.

👍