title: Is there a Has-Only-One constraint in NUnit?
body: <p>I'm finding myself needing a lot of this sort of logic lately:</p>
  
  <pre><code>Assert.That(collection.Items, Has.Member(expected_item));
  Assert.That(collection.Items.Count(), Is.EqualTo(1));
  </code></pre>
  
  <p>I see that NUnit offers <code>Has.Some</code> and <code>Has.All</code>, but I don't see anything like <code>Has.One</code>.  What's the best way to accomplish this without two asserts?</p>