Fix scope chaining + STI

See #9869 and #9929.

The problem arises from the following example:

    class Project < ActiveRecord::Base
      scope :completed, -> { where completed: true }
    end

    class MajorProject < Project
    end

When calling:

    MajorProject.where(tasks_count: 10).completed

This expands to:

    MajorProject.where(tasks_count: 10).scoping {
      MajorProject.completed
    }

However the lambda for the `completed` scope is defined on Project. This
means that when it is called, `self` is Project rather than
MajorProject. So it expands to:

    MajorProject.where(tasks_count: 10).scoping {
      Project.where(completed: true)
    }

Since the scoping was applied on MajorProject, and not Project, this
fails to apply the tasks_count condition.

The solution is to make scoping apply across STI classes. I am slightly
concerned about the possible side-effects of this, but no tests fail and
it seems ok. I guess we'll see.

activerecord/lib/active_record/scoping.rb

@@ -9,11 +9,11 @@ module ActiveRecord
 
     module ClassMethods
       def current_scope #:nodoc:
-        Thread.current["#{self}_current_scope"]
+        Thread.current["#{base_class}_current_scope"]
       end
 
       def current_scope=(scope) #:nodoc:
-        Thread.current["#{self}_current_scope"] = scope
+        Thread.current["#{base_class}_current_scope"] = scope
       end
     end
 

activerecord/lib/active_record/scoping/named.rb

@@ -160,13 +160,8 @@ module ActiveRecord
 
           singleton_class.send(:define_method, name) do |*args|
             if body.respond_to?(:call)
-              scope = extension ? body.call(*args).extending(extension) : body.call(*args)
-
-              if scope
-                default_scoped = scope.default_scoped
-                scope = relation.merge(scope)
-                scope.default_scoped = default_scoped
-              end
+              scope = all.scoping { body.call(*args) }
+              scope = scope.extending(extension) if extension
             else
               scope = body
             end

activerecord/test/cases/scoping/named_scoping_test.rb

@@ -461,7 +461,7 @@ class NamedScopingTest < ActiveRecord::TestCase
   end
 
   def test_subclass_merges_scopes_properly
-    assert_equal 1, SpecialComment.crazy_all.count
+    assert_equal 1, SpecialComment.where(body: 'go crazy').created.count
   end
 
 end

activerecord/test/models/comment.rb

@@ -29,7 +29,6 @@ class Comment < ActiveRecord::Base
 end
 
 class SpecialComment < Comment
-  scope :crazy_all, -> { where(body: 'go crazy').created }
 end
 
 class SubSpecialComment < SpecialComment