Don't rename users table to spree_users if there is a User class defined

If this were to happen, it would most-likely rename the users table for the application, and not the Spree one. The Spree table would never have been created because there would have already been a table called users.

hat-tip: @rohitnair

core/db/migrate/20111007143030_namespace_top_level_models.rb

@@ -41,7 +41,9 @@ def change
     rename_table :taxons,                  :spree_taxons
     rename_table :taxonomies,              :spree_taxonomies
     rename_table :trackers,                :spree_trackers
-    rename_table :users,                   :spree_users
+    unless defined?(User)
+      rename_table :users,                   :spree_users
+    end
     rename_table :variants,                :spree_variants
     rename_table :zones,                   :spree_zones
     rename_table :zone_members,            :spree_zone_members