fix: using with STI models

[michaelkhabarov]

Hello guys. This PR allows to nest ActiveFormModel forms from AR models with STI column.

I have an error trying to use this gem with STI models like that:

class User < ActiveRecord::Base
end

class Client < User
end

class SignUpForm < Client
  include ActiveFormModel
end

SignUpForm.new

Exception:

ActionController::UnfilteredParameters: unable to convert unpermitted parameters to hash

.../actionpack-6.1.4/lib/action_controller/metal/strong_parameters.rb:316:in `to_h'
.../activerecord-6.1.4/lib/active_record/inheritance.rb:278:in `subclass_from_attributes'
.../activerecord-6.1.4/lib/active_record/inheritance.rb:58:in `new'

That's happened because Rails redefines a .new method in active_record/inheritance.rb to find a subclass from attributes passed to Model.new. So #initialize method is not yet called here and we have unpermitted params.

Permitting params in .new instead of #initialize fixes this issue. Please tell me if you see any drawbacks with this approach.

[mokevnin]

Looks good! Resolve conflicts pls and I'll merge it

[michaelkhabarov]

Done