Default Spree.user_class to "LegacyUser"

Don't require it to be set in the initializer anymore.

It's rather common that a store has the initializer still setting the
user_class to "LegacyUser" and the devise initializer re-sets it to
"Spree::User".

core/lib/generators/solidus/install/templates/config/initializers/spree.rb.tt

@@ -79,7 +79,9 @@ Spree::Api::Config.configure do |config|
 end
 <% end -%>
 
-Spree.user_class = <%= (options[:user_class].blank? ? "Spree::LegacyUser" : options[:user_class]).inspect %>
+<% if options[:user_class].present? -%>
+ Spree.user_class = <%= options[:user_class].inspect %>
+<% end -%>
 
 # Rules for avoiding to store the current path into session for redirects
 # When at least one rule is matched, the request path will not be stored

core/lib/spree/core.rb

@@ -28,7 +28,7 @@
 StateMachines::Machine.ignore_method_conflicts = true
 
 module Spree
- mattr_accessor :user_class
+ mattr_accessor :user_class, default: 'Spree::LegacyUser'
 
  def self.user_class
  if @@user_class.is_a?(Class)