arbitrary request header field; the last part of a variable name is the field name converted to lower case with dashes(中划线) replaced by underscores（下划线）
head中后面的变量如果有下划线，nginx默认不支持，需要在nginx.conf里http域里增加underscores_in_headers on;HTTP header
原文地址：http://blog.51cto.com/tenderrain/2135419