Move customer update logic to own service file

The Order class is too big already. We can move more code in the next
commits.

app/models/spree/order.rb

@@ -732,23 +732,7 @@ module Spree
       self.customer ||= find_customer
       self.customer ||= create_customer if require_customer?
 
-      synchronise_customer_email
-    end
-
-    # Update the customer record if the user changed their email address.
-    def synchronise_customer_email
-      if user && customer && user.email != customer.email
-        duplicate = Customer.find_by(email: user.email, enterprise: distributor)
-
-        if duplicate.present?
-          Spree::Order.where(customer_id: duplicate.id).update_all(customer_id: customer.id)
-          Subscription.where(customer_id: duplicate.id).update_all(customer_id: customer.id)
-
-          duplicate.destroy
-        end
-
-        customer.update(email: user.email)
-      end
+      CustomerSyncer.new(self).synchronise_customer_email
     end
 
     def update_adjustment!(adjustment)

app/services/customer_syncer.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+# Create, find and update customer records.
+#
+# P.S.: I almost couldn't resist to call this CustomerService.
+class CustomerSyncer
+  attr_reader :customer, :distributor, :user
+
+  def initialize(order)
+    @customer = order.customer
+    @distributor = order.distributor
+    @user = order.user
+  end
+
+  # Update the customer record if the user changed their email address.
+  def synchronise_customer_email
+    if user && customer && user.email != customer.email
+      duplicate = Customer.find_by(email: user.email, enterprise: distributor)
+
+      if duplicate.present?
+        Spree::Order.where(customer_id: duplicate.id).update_all(customer_id: customer.id)
+        Subscription.where(customer_id: duplicate.id).update_all(customer_id: customer.id)
+
+        duplicate.destroy
+      end
+
+      customer.update(email: user.email)
+    end
+  end
+end