Add accepts_nested_attributes_for support for delegated_type

[cspetterson]

Previously, trying to use nested attributes would raise an error when trying to build polymorphic associations:

Entry.create(entryable_type: 'Message', entryable_attributes: { content: 'Hello world' })
# ArgumentError: Cannot build association `entryable'. Are you trying to build a polymorphic one-to-one association?

Adding this, matches the rails core implementation and prevents using have to add build_#{role} methods every time we introduce a nested association, which was a suggestion on Stack Overflow (https://stackoverflow.com/a/45306832), eg:

def build_attachable(params)
  self.attachable = attachable_type.constantize.new(params)
end

The pull request first appeared in v7.0.0.rc1, here: https://github.com/rails/rails/pull/41717/files

We've since forked this and using it in our app at Zapnito

From the original PR:

Summary

This PR adds nested_attributes_for support to delegated_type, this allows a developer to create and update records easily without needing to write specific methods like:

class Entry < ApplicationRecord
  delegated_type :entryable, types: %w[ Message Comment ]

  def self.create_with_comment(content, creator: Current.user)
    create! entryable: Comment.new(content: content), creator: creator
  end
end

Using nested_attributes_for allows to execute the following:

class Entry < ApplicationRecord
  delegated_type :entryable, types: %w[ Message Comment ]
  accepts_nested_attributes_for :entryable
end

params = { entry: { entryable_type: 'Comment', entryable_attributes: { content: 'Smiling' } } }
entry = Entry.create(params[:entry])

Why

Nested forms based on accepts_nested_attributes_for are very powerful and this is the last piece missing to also be able to use it on Delegated Types.

Question

Since this is a polymorphic belongs_to relationship, what other tests would we like to have? As it is already tested in TestNestedAttributesOnABelongsToAssociation