Add Venmo as payment option on checkout

Resolves issue/ticket:
- solidusio#136

Setting the payment method preference `enable_venmo` to true will now
show a Venmo payment option on checkout for customers whom it is
available to (currently only US).

app/models/solidus_paypal_commerce_platform/payment_method.rb

@@ -12,6 +12,7 @@ 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
 
     def partial_name
       "paypal_commerce_platform"
@@ -73,6 +74,7 @@ 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]
 
       "https://www.paypal.com/sdk/js?#{parameters.to_query}"
     end

spec/models/solidus_paypal_commerce_platform/payment_method_spec.rb

@@ -119,6 +119,23 @@ 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
   end
 
   private