Ignores a default subclass when `becomes(Parent)`

Fixes issue described in #30399: A default value on the
inheritance column prevented `child.becomes(Parent)` to return
an instance of `Parent` as expected, instead it returns an instance
of the default subclass.

The change was introduced by #17169 and it was meant to affect
initialization, alone. Where `Parent.new` is expected to return
an instance of the default subclass.

activerecord/lib/active_record/persistence.rb

@@ -359,7 +359,8 @@ def destroy!
     # Any change to the attributes on either instance will affect both instances.
     # If you want to change the sti column as well, use #becomes! instead.
     def becomes(klass)
-      became = klass.new
+      became = klass.allocate
+      became.send(:initialize)
       became.instance_variable_set("@attributes", @attributes)
       became.instance_variable_set("@mutations_from_database", @mutations_from_database) if defined?(@mutations_from_database)
       became.instance_variable_set("@new_record", new_record?)

activerecord/test/cases/persistence_test.rb

@@ -473,6 +473,22 @@ def test_update_sti_subclass_type
     assert_instance_of Reply, Reply.find(reply.id)
   end
 
+  def test_becomes_default_sti_subclass
+    original_type = Topic.columns_hash["type"].default
+    ActiveRecord::Base.connection.change_column_default :topics, :type, "Reply"
+    Topic.reset_column_information
+
+    reply = topics(:second)
+    assert_instance_of Reply, reply
+
+    topic = reply.becomes(Topic)
+    assert_instance_of Topic, topic
+
+  ensure
+    ActiveRecord::Base.connection.change_column_default :topics, :type, original_type
+    Topic.reset_column_information
+  end
+
   def test_update_after_create
     klass = Class.new(Topic) do
       def self.name; "Topic"; end