title: NUnit Unit Test has "ExpectedException" but still failing on exception
body: <p>I have a unit test that is failing because a <code>System.ArgumentException</code> is being thrown, even though I am <strong>expecting</strong> it and it's deliberate behaviour - what have I missed?</p>
  
  <pre><code>[Test]
  [ExpectedException(typeof(ArgumentException), ExpectedMessage = "Seconds from midnight cannot be more than 86400 in 010100712386401000000012")]
  public void TestParsingCustomReferenceWithInValidSecondsFromMidnight()
  {
      // I am expecting this method to throw an ArgumentException:
      CustomReference.Parse("010100712386401000000012");
  }
  </code></pre>
  
  <p>I've also tried without the <code>ExpectedMessage</code> being set - no difference.</p>