Do not apply a negative adjustment for the default tax zone AND a positive adjustment for the order's tax zone if the tax rate is included in the price

This should fix the Spain + France + Kenya problems outlined here: https://github.com/spree/spree/pull/4327#issuecomment-35364926
Call compute_amount to calculate a TaxRate

This is because it has some extra logic in there which will determine what rate to apply correctly.

Oh, and it's used by Adjustment#update, and it never really does make sense to have two linked parts of your system computing things in two different ways now, does it?
use respond_to rather than rescue NoMethodError in Spree::Calculator

NoMethodErrors can be thrown anywhere down the code path from self.send(method, computable) and this will still tell you to implement the method... even though it's already implemented!

respond_to? won't do such terrible things.
Correct default tax calculation for an order with 60 worth of line items

item totals are now 57.14, tax rate is 5% and combine the two and you get 59.997, which is pretty darn close to 60.
Need to call store_pre_tax_amount before computing non-discounted item's price
reload item in ItemAdjustments to ensure adjustments are present

I was seeing a situation today where a shipment's refunded tax adjustments were not present
Display exclusionary tax amounts on shipping rates
Correct test name in shipment_spec
Shipment#update_amounts now no longer includes included adjustments in its adjustment_total summation

Boy, that was a gigantic pain in the ass to track down. I had to use my ol' trusty blog post: http://ryanbigg.com/2013/06/finding-sql-queries-in-rails/

Originally I had thought that ItemAdjustments was the only place where adjustment_total was being update for items, and so I stared at that for at least an hour trying to figure out where things were going wrong. Then I got mad, found the query that was setting it in the database, tracked it down using the aforementioned blog post, fixed the bloody thing and committed it all within 10 minutes.
Do not attempt to reload an item in ItemAdjustments if it's not persisted