current_scope is not applied for subclasses

The fix is causing a test failure as of now.

```
NamedScopeTest#test_should_not_duplicates_where_values 
[/Users/nsingh/dev/
rails_edge/rails/activerecord/test/cases/named_scope_test.rb:303]:
Expected: ["1=1"]
  Actual: ["1=1", "1=1"]
```

Even though the test is asserting that where values should not be duplicated, the sql that is generated does not have the duplication. So I'm not sure about the validity of the test.

I'll clean up pull request with squashing, change log etc when it is all set to go.

cc @jonleighton @tenderlove