Add Venmo as payment option on checkout

Resolves issues/tickets:
- solidusio#136
- solidusio#137

Setting the preference enable_venmo to true will now show a Venmo button
on checkout for customers whom it is available to.

For testing purposes, the string preference force_buyer_country was
added. You can use this to mock what country the buyer is from, allowing
you to see what funding sources are shown to customers in various
countries.

This preference does not affect checkout on production, environment.
Instead, PayPal will get this information from their ip geolocation.

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

app/models/solidus_paypal_commerce_platform/payment_method.rb

@@ -12,6 +12,8 @@ class PaymentMethod < SolidusSupport.payment_method_parent_class
     preference :display_on_cart, :boolean, default: true
     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"
@@ -73,8 +75,16 @@ 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

@@ -119,6 +119,50 @@ def Struct(data) # rubocop:disable Naming/MethodName
         expect(url.query.split("&")).to include("components=buttons")
       end
     end
+
+    context 'when enable_venmo is false' do
+      let(:order) { instance_double(Spree::Order, checkout_steps: { "foo" => "bar" }) }
+
+      it 'does not include the "enable-funding=venmo" parameter' do
+        expect(url.query.split("&")).not_to include("enable-funding=venmo")
+      end
+    end
+
+    context 'when enable_venmo is true' do
+      let(:order) { instance_double(Spree::Order, checkout_steps: { "foo" => "bar" }) }
+
+      it 'includes "enable-funding=venmo" as a parameter' do
+        paypal_payment_method.preferences.update(enable_venmo: true)
+        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