title: rspec's should_receive on a non-mock object?
body: <p>Is it possible to call <code>should_receive</code> on a non-mock object?</p>
  
  <p>If it is, I'm wondering why this test doesn't pass. <code>SiteStyle</code> has_many <code>images</code> and when I call <code>duplicate!</code> on a site_style, I want the <code>images</code> to call <code>duplicate</code> on themselves as well. The test fails. The actual images get duplicated though. Why is that?</p>
  
  <p><strong>SiteStyle model:</strong></p>
  
  <pre><code>class SiteStyle &lt; ActiveRecord::Base
    belongs_to :site
    belongs_to :user
    validates_presence_of :name, :user
    has_many :images, :as =&gt; :imageable, :order =&gt; "position", :dependent =&gt; :destroy
  
    ...
  
    def duplicate!(user, *args)
      options = args.extract_options!
      to_site ||= Site.find(options[:site_id])
      dup_style = self.class.create :site_id =&gt; to_site.id, :name =&gt; name, :user_id =&gt; user.id
      self.images.each { |image| image.duplicate!(dup_style) }
      dup_style
    end
  
    ...
  end
  </code></pre>
  
  <p><strong>SiteStyleSpec:</strong></p>
  
  <pre><code>require 'spec_helper'
  
  describe SiteStyle do
    ...
  
    describe "duplicate!(user, *args)" do
      before do
        @creator = User.make
        @site = Site.make
        @style = SiteStyle.make :site =&gt; @site, :user =&gt; @creator
      end
  
      ...
  
      it "should have a duplicate of the images" do
        @image = Image.make :imageable_type =&gt; @style.class.name, :imageable_id =&gt; @style.id
        @image.should_receive(:duplicate!).once
        @dup_style = @style.duplicate!(@creator)
      end
    end
  end
  </code></pre>
  
  <p><strong>Error:</strong></p>
  
  <pre><code>Failures:
    ...
  
    2) SiteStyle duplicate!(user, *args) should have a duplicate of the images
       Failure/Error: @image.should_receive(:duplicate!).once
       (#&lt;Image:0xc7b40d8&gt;).duplicate!(any args)
           expected: 1 time
           received: 0 times
       # ./spec/models/site_style_spec.rb:83
  </code></pre>