Perform refund during payment cancellation

Previously the #cancel method relied on the after_create callback
performing the refund. Now it is explicitly called after the refund is
built.

core/app/models/spree/payment/cancellation.rb

@@ -31,7 +31,7 @@ def cancel(payment)
           if response = payment.payment_method.try_void(payment)
             payment.send(:handle_void_response, response)
           else
-            payment.refunds.create!(amount: payment.credit_allowed, reason: refund_reason)
+            payment.refunds.create!(amount: payment.credit_allowed, reason: refund_reason).perform!
           end
         else
           # For payment methods not yet implemeting `try_void`

core/spec/models/spree/order_spec.rb

@@ -127,7 +127,7 @@
       let(:payment) { create(:payment, order: order, amount: payment_amount, state: 'completed') }
 
       before do
-        create(:refund, payment: payment, amount: payment_amount)
+        create(:refund, payment: payment, amount: payment_amount).perform!
       end
 
       it "cancels the order" do
@@ -281,7 +281,7 @@
       order.cancellations.short_ship([order.inventory_units.first])
       expect(order.outstanding_balance).to be_negative
       expect(order.payment_state).to eq('credit_owed')
-      create(:refund, amount: order.outstanding_balance.abs, payment: payment, transaction_id: nil)
+      create(:refund, amount: order.outstanding_balance.abs, payment: payment, transaction_id: nil).perform!
       order.reload
       expect(order.outstanding_balance).to eq(0)
       expect(order.payment_state).to eq('paid')

core/spec/models/spree/payment/cancellation_spec.rb

@@ -51,7 +51,7 @@
             payment.refunds.create!(
               amount: credit_amount,
               reason: Spree::RefundReason.where(name: 'test').first_or_create
-            )
+            ).perform!
           end
 
           it 'only refunds the allowed credit amount' do