title: String interpolation issues
body: <p>I'm trying to figure out why my unit test fails (The third assert below):</p>
  
  <pre><code>var date = new DateTime(2017, 1, 1, 1, 0, 0);
  
  var formatted = "{countdown|" + date.ToString("o") + "}";
  
  //Works
  Assert.AreEqual(date.ToString("o"), $"{date:o}");
  //Works
  Assert.AreEqual(formatted, $"{{countdown|{date.ToString("o")}}}");
  //This one fails
  Assert.AreEqual(formatted, $"{{countdown|{date:o}}}");
  </code></pre>
  
  <p>AFAIK, this should work correctly, but it appears that it doesn't pass the formatting parameter in correctly, it it appears as just <code>{countdown|o}</code> to the code. Any idea why this is failing?</p>