Fix OrderBy on Vouchers.

BulkPrintingAPI/Controllers/VouchersController.cs

             var credentials = await Utils.GetLoginCredentialsFromRequestAsync(HttpContext, _context);
             var result = await Page<Voucher>.GetPageAsync(
                 VouchersForBatchAndVendor(batchId, credentials.Vendor.Id)
-                    .OrderBy(v => new { v.BatchId, v.SequenceNumber }),
+                    .OrderBy(v => v.BatchId)
+                    .ThenBy(v => v.SequenceNumber),
                 page, pageSize);
 
             var now = DateTimeOffset.UtcNow;