InvalidOperationException: Response Content-Length mismatch

FileStreamResult 抛出异常：

InvalidOperationException: Response Content-Length mismatch

解决方案：

Stream 必须 seek begin

伪代码如下：

 Stream ms = new MemoryStream();
 book.Write(ms);
 ms.Seek(0, SeekOrigin.Begin); //seek begin
 return ms;