title: NUnit 3.0 ExpectedResult is always null
body: <p>Migrating tests from NUnit 2.x to 3.x I replaced Result attribute with ExpectedResult for every TestCase. Anyway seems that ExpectedResult does not get the value passed.</p>
  
  <p>My Code:</p>
  
  <pre><code>[TestFixture]
  public class HashToolsTest
  {
          [TestCase("Cantami o diva del pelide Achille l'ira funesta", ExpectedResult = "b4dd7f0b0ca6c25dd46cc096e45158eb")]
           public object MD5_Should_Hash_a_string_according_to_MD5_standards(string toHash) {
                string res = HashTools.MD5(toHash);
                return res;
           }
   }
  </code></pre>
  
  <p>and when i run the tests message is:</p>
  
  <pre><code>Expected: null
  But was:  "b4dd7f0b0ca6c25dd46cc096e45158eb"
  
  at NUnit.Framework.Assert.That(TActual actual, IResolveConstraint expression, String message, Object[] args) 
  at  NUnit.Framework.Assert.AreEqual(Object expected, Object actual)
  </code></pre>
  
  <p>What did I miss?</p>