Backport process_payments_with from Solidus 1.4

There is an issue with the `process_payments_with` method where it will
raise a GatewayError if there are no unprocessed payments when
transitioning from confirm -> complete. It's possible with some of our
payment methods that we already have an authorized payment at that point
and there is no need for any further processing.

This issue was resolved in Solidus 1.4 in the following PR:
solidusio/solidus#1361

app/models/concerns/spree/adyen/order.rb

@@ -10,6 +10,36 @@ def requires_manual_refund?
           payment.source.try(:requires_manual_refund?)
         end
       end
+
+      private
+
+      def process_payments_with(method)
+        if SolidusSupport.solidus_gem_version < Gem::Version.new("1.4.0")
+          process_payments_with_v1_4(method)
+        else
+          super
+        end
+      end
+
+      # Backport from Solidus 1.4 to address the issue resolved in:
+      # https://github.com/solidusio/solidus/pull/1361
+      def process_payments_with_v1_4(method)
+        # Don't run if there is nothing to pay.
+        return true if payment_total >= total
+
+        unprocessed_payments.each do |payment|
+          break if payment_total >= total
+
+          payment.public_send(method)
+
+          if payment.completed?
+            self.payment_total += payment.amount
+          end
+        end
+      rescue Core::GatewayError => e
+        result = !!Spree::Config[:allow_checkout_on_gateway_error]
+        errors.add(:base, e.message) && (return result)
+      end
     end
   end
 end

lib/spree/adyen/engine.rb

@@ -36,7 +36,7 @@ def self.activate
         Spree::CheckoutController.prepend Spree::Adyen::CheckoutController
         Spree::CreditCard.prepend Spree::CreditCard::AdyenToken
         Spree::Payment.prepend Spree::Adyen::Payment
-        Spree::Order.include Spree::Adyen::Order
+        Spree::Order.prepend Spree::Adyen::Order
         Spree::Admin::RefundsController.include Spree::Adyen::Admin::RefundsController
         ::Adyen::API::PaymentService.include Spree::Adyen::PaymentService
         ::Adyen::REST::Response.include Spree::Adyen::REST::Response