Move scoping handling into klass level from relation

I'd like to use this `scoping` handling on klass level to address
unwanted internal scoping issues.

activerecord/lib/active_record/relation.rb

@@ -315,10 +315,7 @@ module ActiveRecord
     # Please check unscoped if you want to remove all previous scopes (including
     # the default_scope) during the execution of a block.
     def scoping
-      previous, klass.current_scope = klass.current_scope(true), self unless @delegate_to_klass
-      yield
-    ensure
-      klass.current_scope = previous unless @delegate_to_klass
+      @delegate_to_klass ? yield : klass._scoping(self) { yield }
     end
 
     def _exec_scope(*args, &block) # :nodoc:

activerecord/lib/active_record/scoping/default.rb

@@ -31,7 +31,14 @@ module ActiveRecord
         #     Post.limit(10) # Fires "SELECT * FROM posts LIMIT 10"
         #   }
         def unscoped
-          block_given? ? relation.scoping { yield } : relation
+          block_given? ? _scoping(relation) { yield } : relation
+        end
+
+        def _scoping(relation) # :nodoc:
+          previous, self.current_scope = current_scope(true), relation
+          yield
+        ensure
+          self.current_scope = previous
         end
 
         # Are there attributes associated with this scope?