Refactor Stock Quantifier to use Enumerable

This allows a developer to preload stock items and stock location,
removing many, many n+1 queries to the database.

core/app/models/spree/stock/quantifier.rb

@@ -9,11 +9,12 @@ class Quantifier
       # @param [StockLocation, Integer] stock_location The stock_location to check inventory in. If unspecified it will check inventory in all available StockLocations
       def initialize(variant, stock_location = nil)
         @variant = variant
-        @stock_items = Spree::StockItem.where(variant_id: variant)
-        if stock_location
-          @stock_items.where!(stock_location: stock_location)
-        else
-          @stock_items.joins!(:stock_location).merge!(Spree::StockLocation.active)
+        @stock_items = variant.stock_items.select do |stock_item|
+          if stock_location
+            stock_item.stock_location == stock_location
+          else
+            stock_item.stock_location.active?
+          end
         end
       end
 
@@ -23,7 +24,7 @@ def initialize(variant, stock_location = nil)
       #   inventory is not tracked on the variant.
       def total_on_hand
         if @variant.should_track_inventory?
-          stock_items.sum(:count_on_hand)
+          stock_items.map(&:count_on_hand).sum
         else
           Float::INFINITY
         end