Allow passing a user class to the test app generator

Currently, solidus_auth_devise is tested with a dummy app that specifies
`Spree::LegacyUser` in its `spree.rb initializer. That's not great, but
there's no way of fixing it if the rake task install does not pass
through the user class option.

The way to use this feature would be in an extension's Rakefile:

```rb
SolidusDevSupport::RakeTasks.install(user_class: "Spree::User")
```

Author: mamhoff

lib/solidus_dev_support/rake_tasks.rb

@@ -7,14 +7,15 @@ module SolidusDevSupport
   class RakeTasks
     include Rake::DSL
 
-    def self.install(*args)
-      new(*args).tap(&:install)
+    def self.install(**args)
+      new(**args).tap(&:install)
     end
 
-    def initialize(root: Dir.pwd)
+    def initialize(root: Dir.pwd, user_class: "Spree::LegacyUser")
       @root = Pathname(root)
       @test_app_path = @root.join(ENV.fetch('DUMMY_PATH', 'spec/dummy'))
       @gemspec = Bundler.load_gemspec(@root.glob("{,*}.gemspec").first)
+      @user_class = user_class
     end
 
     attr_reader :test_app_path, :root, :gemspec
@@ -39,12 +40,12 @@ def install_test_app_task
         # We need to go back to the gem root since the upstream
         # extension:test_app changes the working directory to be the dummy app.
         task :test_app do
-          Rake::Task['extension:test_app'].invoke
+          Rake::Task['extension:test_app'].invoke(@user_class)
           cd root
         end
 
         directory ENV.fetch('DUMMY_PATH', nil) do
-          Rake::Task['extension:test_app'].invoke
+          Rake::Task['extension:test_app'].invoke(@user_class)
         end
       end
     end