Add Client class test

spec/ezpay-invoice/client_spec.rb

@@ -0,0 +1,19 @@
+RSpec.describe EzpayInvoice::Client do
+  describe "#initialize" do
+    context "configuration" do
+      it "reads global configuration by default" do
+        EzpayInvoice.setup { |config| config.merchant_id = 'fake_merchant_id' }
+        client = EzpayInvoice::Client.new
+
+        expect(client.merchant_id).to eq('fake_merchant_id')
+      end
+
+      it "reads instance configuration if options were set" do
+        EzpayInvoice.setup { |config| config.merchant_id = 'global_merchant_id' }
+        client = EzpayInvoice::Client.new({merchant_id: 'instance_merchant_id'})
+
+        expect(client.merchant_id).to eq('instance_merchant_id')
+      end
+    end
+  end
+end