Remove redundant NullSerializer

Use one from ActiveSupport::MessageEncryptor module.

/cc @tamird @chancancode
Also the motivation behind this PR was that in we've used `NullSerializer` from the wrong namespace which broke our patch in places where there was object type comparison.

Yes, we we're doing monkey patching. Yes, it's rather private then public API so you should be careful, but this still is very confusing even for rails-core dev.