Fix convention tests running for gists

Octokit/Models/Request/GistRequest.cs

@@ -1,8 +1,10 @@
 using System;
-using Octokit.Internal;
+using System.Diagnostics;
+using System.Globalization;
 
 namespace Octokit
 {
+    [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class GistRequest : RequestParameters
     {
         public GistRequest(DateTimeOffset since)
@@ -11,5 +13,13 @@ namespace Octokit
         }
 
         public DateTimeOffset Since { get; set; }
+
+        internal string DebuggerDisplay
+        {
+            get
+            {
+                return String.Format(CultureInfo.InvariantCulture, "Since: {0}", Since);
+            }
+        }
     }
 }