Work around a strange piece of Syck behaviour where it checks Model#respond_to? before initializing the object.

Things like YAML.load(YAML.dump(@post)) won't work without this.

adjust to work with instance-based migations
migrated_at can't be null
why must people have last names? it's killing me!

ActiveModel::Errors.to_hash returns plain OrderedHash and used in to_json serialization to properly handle multiple errors per attribute [#5615 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

This reverts commit a9b666b5.

Deprecate handler_class_for_extension as template handlers can be any Ruby object, not necessarily a class.