Removing unessecary call to MapClrMemberNameToJsonFieldName.

The keys in the IDictionary instances in the GetCache are JSON Field
names so the call to MapClrMemberNameToJsonFieldName in
TrySerializeUnknownTypes is unnesecary at best and incorrect at worst
as the input we were passing to that function was not a CLR Member
Name but rather a JSON Field Name.

Octokit/SimpleJson.cs

@@ -1507,7 +1507,7 @@ namespace Octokit
             foreach (KeyValuePair<string, ReflectionUtils.GetDelegate> getter in getters)
             {
                 if (getter.Value != null)
-                    obj.Add(MapClrMemberNameToJsonFieldName(getter.Key), getter.Value(input));
+                    obj.Add(getter.Key, getter.Value(input));
             }
             output = obj;
             return true;