nginx配置中，proxy_set_header可以放在http，server，loaction中

逻辑上讲，下面loaction的配置应该会继承上面的proxy_set_header

但Nginx有一个限制

proxy_set_header directives issued at higher levels are only inherited when no proxy_set_header directives have been issued at a given level.

就是说，当location里写的有其它的proxy_set_header时，则无法继承高层server中定义的proxy_set_header的设置

如果location想继承上面定义的proxy_set_header，则location里不能proxy_set_header指令