Special case JSON _links property.

For some reason, the List Feeds http://developer.github.com/v3/activity/feeds/#list-feeds
API response has a field that starts with an underscore. This also occurs in
the Review Comments API: http://developer.github.com/v3/pulls/comments/#list-comments-in-a-repository
and in the Contents API. http://developer.github.com/v3/repos/contents/#get-the-readme

This commit special cases that one field so we can use the expected CLR
property name. I couldn't find a case where a
JSON response doesn't prefix "links" with an underscore.

Related to #386

Octokit/Http/SimpleJsonSerializer.cs

@@ -23,7 +23,9 @@ namespace Octokit.Internal
         {
             protected override string MapClrMemberNameToJsonFieldName(string clrPropertyName)
             {
-                return clrPropertyName.ToRubyCase();
+                var rubyCased = clrPropertyName.ToRubyCase();
+                if (rubyCased == "links") return "_links"; // Special case for GitHub API
+                return rubyCased;
             }
 
             // This is overridden so that null values are omitted from serialized objects.