AR: new instance of unscoped association ignores unscope

[norydev]

When initializing a new AR object from an unscoped association, the unscope is not taken into account.

See steps to reproduce for an example

Regression Introduced by 17f2f30, it used to work on Rails 5-2

here exactly: https://github.com/rails/rails/blob/6-0-stable/activerecord/lib/active_record/associations/association.rb#L228 reverting to klass.all induces the expected behavior

Steps to reproduce

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "6.0.0"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
    t.boolean :visible, default: false
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
  default_scope -> { where(visible: true) }
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    post.comments << Comment.create!

    new_comment = post.comments.unscope(where: :visible).new

    assert_equal false, new_comment.visible
  end
end

Expected behavior

in new_comment = post.comments.unscope(where: :visible).new, since the default scope is unscoped, I expect new_comment.visible to be false, since it's the default database value for the field

Actual behavior

in new_comment = post.comments.unscope(where: :visible).new, the unscope is ignored and new_comment.visible is true (due to default_scope)

System configuration

Rails version: 6.0.0

Ruby version: 2.6.3