title: How to check for a change from Nil in RSpec
body: <p>So I was pretty sure this was going to work...</p>
  
  <pre><code>expect { file.send(:on_io) {} }.to change{
    file.io.class
  }.from( NilClass ).to( File )
  </code></pre>
  
  <p>but it fails with this message...</p>
  
  <pre><code>result should have initially been NilClass, but was NilClass
  </code></pre>
  
  <p><em>Hu?</em></p>
  
  <p>First off, why does this return as a failure? Secondly, I know normally you can check for nil with <code>be_nil</code> by using the <code>nil?</code> method. Is there some special way to do this with a <code>from().to()</code> in RSpec?</p>