All models used in PATCH verbs should have nullable fields

Octokit/Models/Request/GistUpdate.cs

@@ -14,6 +14,14 @@ namespace Octokit
         }
 
         public string Description { get; set; }
+
+        /// <summary>
+        /// Gets a dictionary of gist files to update.
+        /// </summary>
+        /// <remarks>
+        /// Note: All files from the previous version of the gist are carried over by default if not included in the hash. 
+        /// Deletes can be performed by including the filename with a `null` hash.
+        /// </remarks>
         public IDictionary<string, GistFileUpdate> Files { get; private set; }
 
         internal string DebuggerDisplay