title: FsUnit `should equal` fails on `Some []`
body: <p>When I run this FsUnit test with NUnit 2.6.3,</p>
  
  <pre><code>let f xs = Some (List.map ((+) 2) xs)
  
  [&lt;Test&gt;]
  let test() =
    f []
    |&gt; should equal (Some [])
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>Result Message: 
  Expected: &lt;Some([])&gt;
    But was:  &lt;Some([])&gt;
  Result StackTrace:  
  at FsUnit.TopLevelOperators.should[a,a](FSharpFunc`2 f, a x, Object y)
  </code></pre>
  
  <p>The test fails even though the Expected and Actual in the message are the same. What happened?</p>