Improve ShippingRate#display_price with taxes

[jhawthorn]

This removes about 4 unnecessary queries when calling ShippingRate#display_price.

Previously, the shipment->shipping_rates and shipping_rate->taxes associations were missing inverse of, causing ShippingRateTax#label to re-query the shipping_rate, shipment, and order.

The shipping_rate->taxes association was also queried twice in #display_price, once to determine if there are any taxes, and a second time to determine their labels.

Before

irb(main):002:0> rate.display_price
  Spree::Shipment Load (0.2ms)  SELECT  "spree_shipments".* FROM "spree_shipments" WHERE "spree_shipments"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
  Spree::Order Load (0.3ms)  SELECT  "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
  Spree::ShippingRateTax Exists (0.1ms)  SELECT  1 AS one FROM "spree_shipping_rate_taxes" WHERE "spree_shipping_rate_taxes"."shipping_rate_id" = ? LIMIT ?  [["shipping_rate_id", 1], ["LIMIT", 1]]
  Spree::ShippingRateTax Load (0.2ms)  SELECT "spree_shipping_rate_taxes".* FROM "spree_shipping_rate_taxes" WHERE "spree_shipping_rate_taxes"."shipping_rate_id" = ?  [["shipping_rate_id", 1]]
  Spree::TaxRate Load (0.2ms)  SELECT  "spree_tax_rates".* FROM "spree_tax_rates" WHERE "spree_tax_rates"."deleted_at" IS NULL AND "spree_tax_rates"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
  Spree::ShippingRate Load (0.2ms)  SELECT  "spree_shipping_rates".* FROM "spree_shipping_rates" WHERE "spree_shipping_rates"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
  Spree::Shipment Load (0.1ms)  SELECT  "spree_shipments".* FROM "spree_shipments" WHERE "spree_shipments"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
  Spree::Order Load (0.1ms)  SELECT  "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "$5.00 (+ $0.25 North America)"

After

irb(main):002:0> rate.display_price
  Spree::ShippingRateTax Load (0.2ms)  SELECT "spree_shipping_rate_taxes".* FROM "spree_shipping_rate_taxes" WHERE "spree_shipping_rate_taxes"."shipping_rate_id" = ?  [["shipping_rate_id", 1]]
  Spree::TaxRate Load (0.2ms)  SELECT  "spree_tax_rates".* FROM "spree_tax_rates" WHERE "spree_tax_rates"."deleted_at" IS NULL AND "spree_tax_rates"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
=> "$5.00 (+ $0.25 North America)"

[houndci-bot]

Use amount.zero? instead of amount == 0.

[mamhoff]

Nice work. I'm always impressed by these seemingly small fixes.

[tvdeyen]

Thanks.