title: active model serializers respond_with causes undefined method "to_model"?
body: <p>I have a simple model:</p>
  
  <pre><code>class Receipt
    include ActiveModel::Serialization
    attr_accessor :products
  end
  </code></pre>
  
  <p>and my controller is doing:</p>
  
  <pre><code>def create
    respond_with receipt, :serializer =&gt; ReceiptSerializer
  end
  </code></pre>
  
  <p>and the serializer:</p>
  
  <pre><code>class ReceiptSerializer &lt; ActiveModel::Serializer
    attributes :products
  end
  </code></pre>
  
  <p>and I get:</p>
  
  <blockquote>
  <pre><code> NoMethodError:
     undefined method `to_model' for #&lt;Receipt:0x007f99bcb3b6d8&gt;
  </code></pre>
  </blockquote>
  
  <p>Yet if I change my controller to:</p>
  
  <pre><code>def create
    json = ReceiptSerializer.new(receipt)
    render :json =&gt; json
  end
  </code></pre>
  
  <p>Then everything works fine...  what is happening???</p>
  
  <p>I was using active_model_serializers 0.9.3, but just tried 0.10.2, and the results are the same.</p>