Clear shipment when transitioning to delivery step of

checkout if the current shipping address is not eligible for the
existing shipping method

[Fixes #925]

core/app/models/spree/order.rb

@@ -94,9 +94,7 @@ class Order < ActiveRecord::Base
         end
       end
 
-      before_transition :to => ['delivery'] do |order|
-        order.shipments.each { |s| s.destroy unless s.shipping_method.available_to_order?(order) }
-      end
+      before_transition :to => 'delivery', :do => :remove_invalid_shipments!
 
       after_transition :to => 'complete', :do => :finalize!
       after_transition :to => 'delivery', :do => :create_tax_charge!
@@ -338,6 +336,12 @@ def tax_total
       adjustments.tax.map(&:amount).sum
     end
 
+    # Clear shipment when transitioning to delivery step of checkout if the
+    # current shipping address is not eligible for the existing shipping method
+    def remove_invalid_shipments!
+      shipments.each { |s| s.destroy unless s.shipping_method.available_to_order?(self) }
+    end
+
     # Creates new tax charges if there are any applicable rates. If prices already
     # include taxes then price adjustments are created instead.
     def create_tax_charge!