Fix Calculator Default Fields

For all calculators except the `PercentWithCap` one we have a custom
`calculator_fields` partial. PercentWithCap does not need that - it can
fall back to the default calculator fields partial. However, that
partial does not render currently, because `name` is a symbol that does
not respond to `humanize`.

Adds a system spec as well.

(cherry picked from commit 7e1de2d)

Author: mamhoff

promotions/lib/views/backend/solidus_promotions/admin/calculator_fields/_default_fields.html.erb

@@ -2,5 +2,5 @@
   <%= render "solidus_promotions/admin/shared/preference_fields/#{calculator.preference_type(name)}",
     name: "#{prefix}[calculator_attributes][preferred_#{name}]",
     value: calculator.get_preference(name),
-    label: calculator.class.human_attribute_name("preferred_#{name}", default: I18n.t(name, scope: :spree, default: name.humanize)) %>
+    label: calculator.class.human_attribute_name("preferred_#{name}", default: I18n.t(name, scope: :spree, default: name.to_s.humanize)) %>
 <% end %>

promotions/spec/system/solidus_promotions/backend/promotions_spec.rb

@@ -161,4 +161,15 @@
       expect(page).to have_content("Benefit has been successfully removed!")
     end
   end
+
+  describe "Rendering the promotion edit page with the PercentWithCap calculator" do
+    let(:promotion) { create(:solidus_promotion, name: "My capped promotion", benefits: [benefit]) }
+    let(:benefit) { SolidusPromotions::Benefits::AdjustLineItem.new(calculator:) }
+    let(:calculator) { SolidusPromotions::Calculators::PercentWithCap.new(preferred_percent: 10, preferred_max_amount: 100) }
+
+    it "renders without errors" do
+      visit solidus_promotions.edit_admin_promotion_path(promotion)
+      expect(page).to have_content("My capped promotion")
+    end
+  end
 end