Add force_buyer_country payment method preference for testing

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 see the
different funding sources for different customers and its integration.

This preference is still available on production but has no effect.
It was kept on production to not have differences between the
environments. Otherwise, errors could easily occur on production if
for example, preferred_force_buyer_country method was incorrectly
called.

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

README.md

@@ -172,6 +172,12 @@ $ bin/rails server
 Use Ctrl-C to stop
 ```
 
+### Mocking your buyer country
+PayPal normally looks at your IP geolocation to see where you are located to determine what funding sources are available to you. For example, Venmo is currently only available to US buyers.
+Because of this, you may want to pretend you are from US check that that Venmo is correctly integrated for these customers. To do this, set the payment method's preference of `force_buyer_country` to "US". See more information about preferences above.
+
+This preference has no effect on production.
+
 ### Updating the changelog
 
 Before and after releases the changelog should be updated to reflect the up-to-date status of

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"
@@ -75,6 +76,9 @@ 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]
+      if !Rails.env.production? && options[:force_buyer_country].present?
+        parameters['buyer-country'] = options[:force_buyer_country]
+      end
 
       "https://www.paypal.com/sdk/js?#{parameters.to_query}"
     end

spec/models/solidus_paypal_commerce_platform/payment_method_spec.rb

@@ -129,6 +129,31 @@ 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
+      it 'does not include the "buyer-country" parameter' do
+        expect(url.query.split('&')).not_to include(match 'buyer-country')
+      end
+    end
+
+    context 'when force_buyer_country is "US"' do
+      before { paypal_payment_method.preferences.update(force_buyer_country: 'US') }
+
+      it 'includes "buyer-country=US" as a parameter' do
+        expect(url.query.split('&')).to include('buyer-country=US')
+      end
+    end
+
+    context 'when force_buyer_country is "US" but the environment is production' do
+      before {
+        allow(Rails.env).to receive(:production?).and_return(true)
+        paypal_payment_method.preferences.update(force_buyer_country: 'US')
+      }
+
+      it 'includes "buyer-country=US" as a parameter' do
+        expect(url.query.split('&')).not_to include(match 'buyer-country')
+      end
+    end
   end
 
   private