Add make_default method on AddPaymentSourcesToWallet class

Move the logic to make the last payment source as default outside the 
add_to_wallet method to take advantage if someone wants to change only 
the make default behavior.

core/app/models/spree/wallet/add_payment_sources_to_wallet.rb

@@ -24,16 +24,21 @@ def add_to_wallet
       # add valid sources to wallet and optionally set a default
       if sources.any?
         # arbitrarily sort by id for picking a default
-        wallet_payment_sources = sources.sort_by(&:id).map do |source|
+        sources.sort_by(&:id).each do |source|
           order.user.wallet.add(source)
         end
 
-        order.user.wallet.default_wallet_payment_source =
-          wallet_payment_sources.last
+        make_default
       end
     end
   end
 
+  protected
+
+  def make_default
+    order.user.wallet.default_wallet_payment_source = order.user.wallet_payment_sources.last
+  end
+
   private
 
   attr_reader :order

core/spec/models/spree/wallet/add_payment_sources_to_wallet_spec.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Spree::Wallet::AddPaymentSourcesToWallet, type: :model do
+  let(:order) { create(:order_ready_to_complete) }
+
+  describe '#add_to_wallet' do
+    subject { described_class.new(order) }
+
+    it 'saves the payment source' do
+      expect { subject.add_to_wallet }.to change {
+        order.user.wallet.wallet_payment_sources.count
+      }.by(1)
+    end
+  end
+end