title: Why Path.Combine doesn't add the Path.DirectorySeparatorChar after the drive designator?
body: <pre><code>var actual = Path.Combine("c:", "filename");
  var expected = @"c:\filename";
  Assert.AreEqual(expected, actual);
  </code></pre>
  
  <p>Result</p>
  
  <pre><code>{Assert.AreEqual failed. Expected:&lt;c:\filename&gt;. Actual:&lt;c:filename&gt;.
  </code></pre>
  
  <p>Why?</p>