Find customers by user id as well as email

Also fixing up a spec which became brittle due to too much mocking.

app/models/spree/order.rb

@@ -718,7 +718,8 @@ module Spree
     end
 
     def find_customer
-      Customer.of(distributor).find_by(email: email_for_customer)
+      user&.customers&.of(distributor)&.first ||
+        Customer.of(distributor).find_by(email: email_for_customer)
     end
 
     def create_customer

spec/models/spree/order/callbacks_spec.rb

@@ -21,10 +21,10 @@ describe Spree::Order do
 
   context "#save" do
     context "when associated with a registered user" do
-      let(:user) { double(:user, email: "test@example.com") }
+      let(:user) { Spree::User.new(email: "test@example.com") }
 
       before do
-        allow(order).to receive_messages user: user
+        order.user = user
       end
 
       it "should assign the email address of the user" do

spec/models/spree/order_spec.rb

@@ -975,8 +975,6 @@ describe Spree::Order do
       end
 
       it "recognises users with changed email address" do
-        pending "#9002"
-
         order.update!(state: "complete")
 
         # Change email instantly without confirmation via Devise: