Fix that saving completed order would delete store credit adjustments

Because when the order is completed the credits are consumed, the
user no longer has sufficient credit to cover the adjustments so
they were being destroyed.

app/models/spree/order_decorator.rb

@@ -7,7 +7,7 @@ module Spree
     # admin app when creating a new order from the admin console
     # In that case, we create an order before assigning a user
     before_save :process_store_credit, :if => "self.user.present? && @store_credit_amount"
-    after_save :ensure_sufficient_credit, :if => "self.user.present?"
+    after_save :ensure_sufficient_credit, :if => "self.user.present? && !self.completed?"
 
     validates_with StoreCreditMinimumValidator