Warn in case you are using the previous session schema, closes #386

lib/devise/rails/warden_compat.rb

@@ -20,6 +20,13 @@ class Warden::SessionSerializer
 
   def deserialize(keys)
     klass, id = keys
+
+    if klass.is_a?(Class)
+      raise "Devise changed how it stores objects in session. If you are seeing this message, " <<
+        "you can fix it by changing one character in your cookie secret, forcing all previous " <<
+        "cookies to expire, or cleaning up your database sessions if you are using a db store."
+    end
+
     klass.constantize.find(:first, :conditions => { :id => id })
   rescue NameError => e
     if e.message =~ /uninitialized constant/