fixed missing language feature: 'default' literal

The default literal was added in C# v7.1

Recurly/IPager.cs

@@ -10,6 +10,6 @@ public interface IPager<T> : IEnumerable<T>
         List<T> Data { get; }
 
         IPager<T> FetchNextPage();
-        Task<IPager<T>> FetchNextPageAsync(CancellationToken cancellationToken = default);
+        Task<IPager<T>> FetchNextPageAsync(CancellationToken cancellationToken = default(CancellationToken));
     }
 }