Add force_country preference for testing purposes

Resolves issue/ticket:
- #137

Usually on checkout, PayPal will look at the buyer's location by their
ip and determine which funding sources are available to them. For
example, allow Venmo as an option for US buyers, but not for others.

With this new preference, you can set a country to override the buyer
location. Allowing you on non-production environments to the different
funding sources for different customers and their integration.

More information:
https://developer.paypal.com/docs/business/javascript-sdk/javascript-sdk-configuration/#buyer-country

app/models/solidus_paypal_commerce_platform/payment_method.rb

@@ -13,6 +13,7 @@ class PaymentMethod < SolidusSupport.payment_method_parent_class
     preference :display_on_product_page, :boolean, default: true
     preference :display_credit_messaging, :boolean, default: true
     preference :enable_venmo, :boolean, default: false
+    preference :force_buyer_country, :string
 
     def partial_name
       "paypal_commerce_platform"
@@ -75,8 +76,15 @@ def javascript_sdk_url(order: nil, currency: nil)
 
       parameters[:shipping_preference] = 'NO_SHIPPING' if step_names.exclude? 'delivery'
       parameters['enable-funding'] = 'venmo' if options[:enable_venmo]
+      parameters['buyer-country'] = options[:force_buyer_country] if add_buyer_country_to_url?
 
       "https://www.paypal.com/sdk/js?#{parameters.to_query}"
     end
+
+    private
+
+    def add_buyer_country_to_url?
+      options[:force_buyer_country].present? && !Rails.env.production?
+    end
   end
 end

spec/models/solidus_paypal_commerce_platform/payment_method_spec.rb

@@ -136,6 +136,33 @@ def Struct(data) # rubocop:disable Naming/MethodName
         expect(url.query.split("&")).to include("enable-funding=venmo")
       end
     end
+
+    context 'when force_buyer_country is an empty string' do
+      let(:order) { instance_double(Spree::Order, checkout_steps: { "foo" => "bar" }) }
+
+      it 'does not include the "buyer-country" parameter' do
+        expect(url.query.split("&")).not_to include("buyer-country")
+      end
+    end
+
+    context 'when force_buyer_country is "US"' do
+      let(:order) { instance_double(Spree::Order, checkout_steps: { "foo" => "bar" }) }
+
+      it 'includes "buyer-country=US" as a parameter' do
+        paypal_payment_method.preferences.update(force_buyer_country: 'US')
+        expect(url.query.split("&")).to include("buyer-country=US")
+      end
+    end
+
+    context 'when force_buyer_country is "US" and environment is production' do
+      let(:order) { instance_double(Spree::Order, checkout_steps: { "foo" => "bar" }) }
+
+      it 'does not include the "buyer-country" parameter' do
+        paypal_payment_method.preferences.update(force_buyer_country: 'US')
+        allow(Rails.env).to receive(:production?).and_return(true)
+        expect(url.query.split("&")).not_to include("buyer-country")
+      end
+    end
   end
 
   private