addressed a bunch of the Extensions changes

Octokit/Helpers/SerializeNullAttribute.cs

@@ -2,6 +2,10 @@
 
 namespace Octokit.Internal
 {
+    /// <summary>
+    /// Indicate to the serializer that this property or field
+    /// should be included, even when currently set to `null`
+    /// </summary>
     [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
     public sealed class SerializeNullAttribute : Attribute
     {