test: add spec for Spree::CreditCard#brand= setter to verify card type reformatting

spec/models/spree/credit_card_spec.rb

@@ -198,6 +198,15 @@ RSpec.describe Spree::CreditCard do
       end
     end
 
+    context "#brand=" do
+      let(:credit_card) { build(:credit_card) }
+
+      it "delegates to #cc_type= and reformats the card type" do
+        credit_card.brand = 'mastercard'
+        expect(credit_card.cc_type).to eq 'master'
+      end
+    end
+
     context "on save" do
       it "converts the card type format" do
         expect_any_instance_of(described_class).to receive(:reformat_card_type!).