STI should identify itself inside named_scope

[#1570 state:resovled]

Signed-off-by: José Valim <jose.valim@gmail.com>

activerecord/lib/active_record/named_scope.rb

@@ -89,7 +89,11 @@ def named_scope(name, options = {}, &block)
             when Hash
               options
             when Proc
-              options.call(*args)
+              if self.model_name != parent_scope.model_name
+                parent_scope.instance_exec(*args, &options)
+              else
+                options.call(*args)
+              end
           end, &block)
         end
         (class << self; self end).instance_eval do