Refresh the products cache on stock change

This ensures the cache is up-to-date when a user accesses the shopfront
and we use the appropriate stock levels.

Besides, this makes the (very annoying) ProductsCacheIntegrityCheckerJob
succeed and thus Bugsnag won't be flooded with failures anymore.

As for the implementation of this. I'm not happy at all. It's disgusting
to use `class_eval` for this but it's a good trade-off to deliver this
fix ASAP. Katuma's been experiencing this error far too long
🙈. I was working on a better implementation that required
a bit of refactoring but it was taking too much time. I'll open another
PR with it soon.

app/models/spree/stock_item_decorator.rb

@@ -0,0 +1,7 @@
+Spree::StockItem.class_eval do
+  after_save :refresh_products_cache
+
+  def refresh_products_cache
+    OpenFoodNetwork::ProductsCache.variant_changed(variant)
+  end
+end

spec/models/spree/stock_item_spec.rb

@@ -0,0 +1,10 @@
+require 'spec_helper'
+
+describe Spree::StockItem do
+  let!(:variant) { create(:variant) }
+
+  it 'refreshes the products cache on save' do
+    expect(OpenFoodNetwork::ProductsCache).to receive(:variant_changed).with(variant)
+    variant.on_hand = -2
+  end
+end

spec/models/spree/variant_spec.rb

@@ -166,7 +166,8 @@ module Spree
       let(:variant) { create(:variant) }
 
       it "refreshes the products cache on save" do
-        expect(OpenFoodNetwork::ProductsCache).to receive(:variant_changed).with(variant)
+        # When creating the variant both the Variant and StockItem callbacks get executed
+        expect(OpenFoodNetwork::ProductsCache).to receive(:variant_changed).with(variant).twice
         variant.sku = 'abc123'
         variant.save
       end
@@ -182,7 +183,8 @@ module Spree
 
         it "refreshes the products cache for the entire product on save" do
           expect(OpenFoodNetwork::ProductsCache).to receive(:product_changed).with(product)
-          expect(OpenFoodNetwork::ProductsCache).to receive(:variant_changed).never
+          # The StockItem callback is still executed
+          expect(OpenFoodNetwork::ProductsCache).to receive(:variant_changed).once
           master.sku = 'abc123'
           master.save
         end