$http_name arbitrary request header field; the last part of a variable name is the field name converted to lower case with dashes replaced by underscores,
如果值中有横杠会被替换为下划线。
参考:https://www.nginx.com/wp-content/uploads/2017/01/nginx-modules-reference-r11.pdf
http://mailman.nginx.org/pipermail/nginx/2014-March/042905.html