title: Undefined method `to_hash' while saving with reform gem
body: <p>I got a problem. I'm using reform gem in my Rails application with some complex forms. Unfortunately I stuck while creating a form. That's my code:</p>
  
  <pre><code>class Order::Form &lt; Reform::Form
    include Coercion
    include Composition
    include Reform::Form::ActiveRecord
  
    property :subscription, on: :order
    property :due_date, type: Date, on: :order
    property :start_fee, numericality: true, on: :order
  
    collection :products, populate_if_empty: Product, on: :order do
      property :type
      property :budget
    end
  
    property :campaign, on: :order do
      property :customer_id
  
      property :customer do
        property :id
        property :organization_number
      end
    end
  
    model :order
  
    def persist!(params)
      if validate(params)
        binding.pry
        # begin
          ::ActiveRecord::Base.transaction do
            save do |data, map|
              ...
            end
          end
        # rescue Exception
        #   false
        # end
      end
    end
  end
  </code></pre>
  
  <p>And when I'm trying to save I recieve this error:</p>
  
  <pre><code>NoMethodError - undefined method `to_hash' for #&lt;#&lt;Class:0x007f984ddef6d8&gt;:0x007f984a65a4&gt;
  </code></pre>
  
  <p>I wonder if I correctly configured Composition with nested models.</p>