ActiveRecord::Base#hash should differ between classes

Prior to this change, we would get collisions if Active Record objects
of different classes with the same ID were used as keys of the same
hash. It bothers me slightly that we have to allocate inside of this
method, but Ruby doesn't provide any way to hash multiple values without
allocation

activerecord/lib/active_record/core.rb

@@ -432,7 +432,7 @@ def ==(comparison_object)
     #   [ Person.find(1), Person.find(2), Person.find(3) ] & [ Person.find(1), Person.find(4) ] # => [ Person.find(1) ]
     def hash
       if id
-        id.hash
+        [self.class, id].hash
       else
         super
       end

activerecord/test/cases/base_test.rb

@@ -1504,6 +1504,10 @@ def test_default_values_are_deeply_dupped
     assert_not_equal Post.new.hash, Post.new.hash
   end
 
+  test "records of different classes have different hashes" do
+    assert_not_equal Post.new(id: 1).hash, Developer.new(id: 1).hash
+  end
+
   test "resetting column information doesn't remove attribute methods" do
     topic = topics(:first)