Cleaning up ActiveModel::Dirty tests

* Clarifying what the #changed method returns
* Adding tests to describe what the #changed_attributes returns
* Updating test name based on pull request comment
* Moving the test lower in the file per pull request comment

activemodel/test/cases/dirty_test.rb

@@ -46,7 +46,7 @@ def save
     assert @model.name_changed?
   end
 
-  test "list of changed attributes" do
+  test "list of changed attribute keys" do
     assert_equal [], @model.changed
     @model.name = "Paul"
     assert_equal ['name'], @model.changed
@@ -106,6 +106,17 @@ def save
     assert_equal [nil, "Jericho Cane"], @model.previous_changes['name']
   end
 
+  test "previous value is preserved when changed after save" do
+    assert_equal({}, @model.changed_attributes)
+    @model.name = "Paul"
+    assert_equal({ "name" => nil }, @model.changed_attributes)
+
+    @model.save
+
+    @model.name = "John"
+    assert_equal({ "name" => "Paul" }, @model.changed_attributes)
+  end
+
   test "changing the same attribute multiple times retains the correct original value" do
     @model.name = "Otto"
     @model.save