Include Refund Total in Outstanding Balance calculation

Based on the regression test added in solidusio#1926 this change fixes
that spec by including refund_total in outstanding_balance
calculation if the order has not been canceled.

core/app/models/spree/order.rb

@@ -368,7 +368,7 @@ def outstanding_balance
       if state == 'canceled'
         -1 * payment_total
       else
-        total - payment_total
+        total - payment_total - refund_total
       end
     end
 

core/spec/models/spree/order/payment_spec.rb

@@ -188,9 +188,9 @@ module Spree
 
         context "for non-canceled orders" do
           it 'should incorporate refund reimbursements' do
-            # Order Total - Payment Total
-            # 110 - 100 = 10
-            expect(order.outstanding_balance).to eq 10
+            # Order Total - Payment Total - Refund Total
+            # 110 - 100 - 10 = 0
+            expect(order.outstanding_balance).to eq 0
           end
         end
       end

core/spec/models/spree/order_spec.rb

@@ -215,7 +215,7 @@
       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)
-      order.reload
+      order.update!
       expect(order.outstanding_balance).to eq(0)
       expect(order.payment_state).to eq('paid')
     end