Removes caching from ActiveRecord::Core::ClassMethods#relation

The #relation method gets called in four places and the return value was instantly cloned in three of them. The only place that did not clone was ActiveRecord::Scoping::Default::ClassMethods#unscoped. This introduced a bug described in rails#5667 and should really clone the relation, too. This means all four places would clone the relation, so it doesn't make a lot of sense caching it in the first place.

The four places with calls to relations are:

activerecord/lib/active_record/scoping/default.rb:110:in `block in build_default_scope'"
activerecord/lib/active_record/scoping/default.rb:42:in `unscoped'"
activerecord/lib/active_record/scoping/named.rb:38:in `scoped'"
activerecord/lib/active_record/scoping/named.rb:52:in `scope_attributes'"

Conflicts:

	activerecord/lib/active_record/core.rb

activerecord/lib/active_record/scoping/named.rb

@@ -34,7 +34,7 @@ def scoped(options = nil)
             if current_scope
               current_scope.clone
             else
-              scope = relation.clone
+              scope = relation
               scope.default_scoped = true
               scope
             end
@@ -48,7 +48,7 @@ def scope_attributes # :nodoc:
           if current_scope
             current_scope.scope_for_create
           else
-            scope = relation.clone
+            scope = relation
             scope.default_scoped = true
             scope.scope_for_create
           end

activerecord/test/cases/relations_test.rb

@@ -1088,6 +1088,10 @@ def test_unscoped_block_style
     assert_equal 'honda', FastCar.unscoped { FastCar.order_using_old_style.limit(1).first.name}
   end
 
+  def test_unscoped_relation_clones
+    assert_not_equal CoolCar.unscoped.object_id, CoolCar.unscoped.object_id
+  end
+
   def test_intersection_with_array
     relation = Author.where(:name => "David")
     rails_author = relation.first