Extract balance-specific serializer

So we only show the customer balance where really needed. Aggregating
the balance can be costly. Also, we avoid defensive coding.

spec/serializers/api/admin/customer_with_balance_serializer_spec.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Api::Admin::CustomerWithBalanceSerializer do
+  let(:serialized_customer) { described_class.new(customer) }
+
+  describe '#balance' do
+    let(:customer) { double(Customer, balance_value: 1.2) }
+    let(:money) { instance_double(Spree::Money, to_s: "$1.20") }
+
+    before do
+      allow(Spree::Money).to receive(:new).with(1.2, currency: "AUD") { money }
+    end
+
+    it 'returns the balance_value as a money amount' do
+      expect(serialized_customer.balance).to eq("$1.20")
+    end
+  end
+
+  describe '#balance_status' do
+    context 'when the balance_value is positive' do
+      let(:customer) { double(Customer, balance_value: 1) }
+
+      it 'returns credit_owed' do
+        expect(serialized_customer.balance_status).to eq("credit_owed")
+      end
+    end
+
+    context 'when the balance_value is negative' do
+      let(:customer) { double(Customer, balance_value: -1) }
+
+      it 'returns credit_owed' do
+        expect(serialized_customer.balance_status).to eq("balance_due")
+      end
+    end
+
+    context 'when the balance_value is zero' do
+      let(:customer) { double(Customer, balance_value: 0) }
+
+      it 'returns credit_owed' do
+        expect(serialized_customer.balance_status).to eq("")
+      end
+    end
+  end
+end