Always reset changed attributes in becomes

When ```becomes``` changes @attributes it should also change
@changed_attributes. Otherwise we'll experience a kind of split head situation
where attributes are coming from ```self```, but changed_attributes is coming
from ```klass.new```. This affects the inheritance_colmn as it's changed by new
for example.

Fixes #16881

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Fixed ActiveRecord::Relation#becomes! and changed_attributes issues for type column
+
+    Fixes #17139.
+
+    *Miklos Fazekas*
+
 *   Fixed several edge cases which could result in a counter cache updating
     twice or not updating at all for `has_many` and `has_many :through`.
 

activerecord/lib/active_record/persistence.rb

@@ -208,7 +208,8 @@ module ActiveRecord
     def becomes(klass)
       became = klass.new
       became.instance_variable_set("@attributes", @attributes)
-      became.instance_variable_set("@changed_attributes", @changed_attributes) if defined?(@changed_attributes)
+      changed_attributes = @changed_attributes if defined?(@changed_attributes)
+      became.instance_variable_set("@changed_attributes", changed_attributes || {})
       became.instance_variable_set("@new_record", new_record?)
       became.instance_variable_set("@destroyed", destroyed?)
       became.instance_variable_set("@errors", errors)

activerecord/test/cases/inheritance_test.rb

@@ -121,6 +121,12 @@ class InheritanceTest < ActiveRecord::TestCase
     assert_kind_of Cabbage, cabbage
   end
 
+  def test_becomes_and_change_tracking_for_inheritance_columns
+    cucumber = Vegetable.find(1)
+    cabbage = cucumber.becomes!(Cabbage)
+    assert_equal ['Cucumber', 'Cabbage'], cabbage.custom_type_change
+  end
+
   def test_alt_becomes_bang_resets_inheritance_type_column
     vegetable = Vegetable.create!(name: "Red Pepper")
     assert_nil vegetable.custom_type