Fix displaying of deleted variants in admin panel

Because of the default scope added in
466251e,
the `discarded` scope currently operates on the default `kept` scope and
returns an empty set.

This fixes the scope for checking the `discarded` variants as
suggested here https://github.com/jhawthorn/discard#default-scope, to
correctly display the "deleted variants" filter in the admin variants
page.

backend/app/views/spree/admin/variants/_table_filter.html.erb

@@ -11,7 +11,7 @@
       </div>
     </div>
 
-    <% if product.variants.discarded.any? %>
+    <% if product.variants.with_discarded.discarded.any? %>
       <div class="col-2">
         <div class="field checkbox">
           <label>

backend/app/views/spree/admin/variants/index.html.erb

@@ -14,7 +14,7 @@
   <% end %>
 <% end %>
 
-<% if @variants.any? || @product.variants.discarded.any? %>
+<% if @product.variants.with_discarded.any? %>
   <%= render "table_filter", product: @product %>
   <%= render "table", variants: @variants %>
 <% else %>

backend/spec/features/admin/products/variant_spec.rb

@@ -48,6 +48,28 @@
         end
       end
     end
+
+    context 'displaying discarded variants' do
+      let!(:existing_variant) { create(:variant, sku: 'existing_variant_sku', product: product) }
+      let!(:discarded_variant) { create(:variant, sku: 'discarded_variant_sku', product: product) }
+
+      before { discarded_variant.discard! }
+
+      it 'does not display deleted variants by default' do
+        visit spree.admin_product_variants_path(product)
+
+        expect(page).to have_content(existing_variant.sku)
+        expect(page).not_to have_content(discarded_variant.sku)
+      end
+
+      it 'allows to display deleted variants with a filter' do
+        visit spree.admin_product_variants_path(product)
+        check 'Show Deleted Variants'
+        click_button 'search'
+
+        expect(page).to have_content(discarded_variant.sku)
+      end
+    end
   end
 
   context "editing existent variant" do