Fix optional excise attribute

spec/yookassa/entity/product_spec.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+RSpec.describe Yookassa::Entity::Product do
+  let(:attributes) do
+    {
+      description: "Fuel",
+      quantity: 1,
+      amount: { value: 100, currency: "RUB" },
+      vat_code: 1
+    }
+  end
+
+  it "defines excise as an optional numeric attribute" do
+    expect(described_class.new(**attributes, excise: "12.34").excise).to eq(12.34)
+    expect(described_class.new(**attributes).excise).to be_nil
+  end
+end