Extension template: Use Spree::User for dummy app

The Gemfile that's generated for testing extensions contains
solidus_auth_devise, which sets the user class for the app to
Spree::User. The generator in this Rakefile should be set up to do the
same.

Author: mamhoff

lib/solidus_dev_support/templates/extension/Rakefile

@@ -2,6 +2,6 @@
 
 require "bundler/gem_tasks"
 require 'solidus_dev_support/rake_tasks'
-SolidusDevSupport::RakeTasks.install
+SolidusDevSupport::RakeTasks.install(user_class: "Spree::User")
 
 task default: 'extension:specs'