标头出现在IIS日志中但不在Request.Headers中

Question

我们的FWLB正在发送X-Forwarded-Proto标头，该标头在IIS日志中可见：

#Fields: date time cs-method ... X-Forwarded-For X-Forwarded-Proto
2017-06-05 13:34:35 GET ... 1.2.3.4 http

但是从C＃开始它是不可见的：

var output = $"X-Forwarded-Proto is {Request.Headers["X-Forwarded-Proto"]} headers are: {string.Join(", ", Request.Headers)}";

输出

X-Forwarded-Proto is  headers are: Connection=close&Host=1.1.1.1

为什么在C＃中找不到标题？