Use Spree user class handle

app/models/solidus_paypal_braintree/customer.rb

@@ -1,4 +1,4 @@
 class SolidusPaypalBraintree::Customer < ApplicationRecord
-  belongs_to :user, class_name: "Spree::User"
+  belongs_to :user, class_name: Spree::UserClassHandle.new
   has_many :sources, class_name: "SolidusPaypalBraintree::Source", inverse_of: :customer
 end

app/models/solidus_paypal_braintree/source.rb

@@ -4,7 +4,7 @@ class Source < ApplicationRecord
     APPLE_PAY = "ApplePayCard"
     CREDIT_CARD = "CreditCard"
 
-    belongs_to :user, class_name: "Spree::User"
+    belongs_to :user, class_name: Spree::UserClassHandle.new
     belongs_to :payment_method, class_name: 'Spree::PaymentMethod'
     has_many :payments, as: :source, class_name: "Spree::Payment"
 

spec/models/solidus_paypal_braintree/transaction_import_spec.rb

@@ -55,7 +55,7 @@
     end
 
     context 'order has a user' do
-      let(:user) { Spree::User.new }
+      let(:user) { Spree.user_class.new }
       let(:order) { Spree::Order.new user: user }
 
       it 'associates user to the source' do
@@ -70,7 +70,7 @@
     it { is_expected.to be_nil }
 
     context 'when order has a user' do
-      let(:user) { Spree::User.new }
+      let(:user) { Spree.user_class.new }
       let(:order) { Spree::Order.new user: user }
 
       it { is_expected.to eq user }