Allow variant inventory quantity

lib/shopify_api/resources/variant.rb

@@ -26,10 +26,9 @@ def old_inventory_quantity=(new_value)
       super
     end
 
-    def serializable_hash(options = {})
-      super(options).tap do |resource|
-        (resource['variant'] || resource).except!('inventory_quantity', 'old_inventory_quantity')
-      end
+    def save
+      attributes.except!('inventory_quantity', 'old_inventory_quantity')
+      super
     end
 
     private

test/product_test.rb

@@ -65,12 +65,12 @@ def test_price_range_when_has_different_price
     assert_equal('100.00 - 199.00', @product.price_range)
   end
 
-  def test_read_only_variant_inventory_fields_not_present_in_2020_01
+  def test_read_only_variant_inventory_fields_not_removed_in_2020_01
     ShopifyAPI::Base.api_version = '2020-01'
     refresh_product(api_version: ShopifyAPI::Base.api_version)
 
     variant = @product.variants.first
-    refute(variant.as_json.include?('inventory_quantity'))
+    assert(10, variant.inventory_quantity)
   end
 
   def test_deprecated_inventory_fields_are_removed_in_2020_01

test/variant_test.rb

@@ -38,8 +38,7 @@ def test_delete_variant
   def test_read_only_inventory_quantity
     ShopifyAPI::Base.api_version = '2020-01'
     refresh_variant(api_version: ShopifyAPI::Base.api_version)
-    assert(@variant.inventory_quantity, 10)
-    refute(@variant.as_json.include?('inventory_quantity'))
+    assert(10, @variant.inventory_quantity)
   end
 
   def test_setting_variant_inventory_quantity_adjustment_fails_in_the_unstable_api