Allow control over Venmo PayPal financing option

Resolves issue/ticket:
- solidusio#136

If a transaction meets Venmo prerequisites [1], then Venmo button
should appear already on checkout, cart and product page depending
on payment method preferences.

These changes is to give more control to the developer so that they can
disable Venmo by setting the `enable_venmo` to false.
https://developer.paypal.com/docs/business/javascript-sdk/javascript-sdk-configuration/#disable-funding

As it Venmo may have already been showing for developers, the default
was set to true.

The enable-funding option is now also sent over to ensure that the Venmo
button gets rendered if eligible:
https://developer.paypal.com/docs/business/javascript-sdk/javascript-sdk-configuration/#enable-funding
https://developer.paypal.com/docs/business/checkout/pay-with-venmo/integrate-pay-with-venmo/#always-request-venmo

[1] https://developer.paypal.com/docs/business/checkout/pay-with-venmo/#eligibility

README.md

@@ -120,6 +120,18 @@ With product and cart page checkout, the user is directed to the checkout confir
 
 PayPals API does not allow for admin-side payments. Instead, backend users taking payments for customers will need to use the PayPal Virtual Terminal to take payments. [More info is available on the PayPal website.](https://www.paypal.com/merchantapps/appcenter/acceptpayments/virtualterminal?locale.x=en_US)
 
+## Venmo
+Venmo is currently available to US merchants and buyers. There are also other [prequisites](https://developer.paypal.com/docs/business/checkout/pay-with-venmo/#eligibility).
+
+If the transaction supports Venmo then a button should appear for it on checkout, cart and product page, depending on your `Payment Method` preferences.
+
+If you wish to disable Venmo, then set your `Payment Methods`'s `enable_venmo` preference to `false`. See more about preferences([Configuration](#configuration)) below.
+
+## Configuration
+The easiest way to change the `Payment Method`'s preferences is through admin: `Settings > Payments > "PayPal Commerce Platform" > Edit`.
+
+See more about preferences [here](https://guides.solidus.io/developers/preferences/add-model-preferences.html#access-your-preferences)/
+
 ## Development
 
 ### Testing the extension

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: true
 
     def partial_name
       "paypal_commerce_platform"
@@ -73,6 +74,8 @@ 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['disable-funding'] = 'venmo' unless options[:enable_venmo]
 
       "https://www.paypal.com/sdk/js?#{parameters.to_query}"
     end

spec/models/solidus_paypal_commerce_platform/payment_method_spec.rb

@@ -113,6 +113,30 @@ def Struct(data) # rubocop:disable Naming/MethodName
         expect(url.query.split("&")).to include("components=buttons")
       end
     end
+
+    context 'when enable_venmo is false' do
+      before { paypal_payment_method.preferences.update(enable_venmo: false) }
+
+      it 'does not include "enable-funding=venmo" as a parameter' do
+        expect(url.query.split('&')).not_to include('enable-funding=venmo')
+      end
+
+      it 'includes "disable-funding=venmo" as a parameter' do
+        expect(url.query.split('&')).to include('disable-funding=venmo')
+      end
+    end
+
+    context 'when enable_venmo is true' do
+      before { paypal_payment_method.preferences.update(enable_venmo: true) }
+
+      it 'includes "enable-funding=venmo" as a parameter' do
+        expect(url.query.split('&')).to include('enable-funding=venmo')
+      end
+
+      it 'does not include "disable-funding=venmo" as a parameter' do
+        expect(url.query.split('&')).not_to include('disable-funding=venmo')
+      end
+    end
   end
 
   private