request help: The proxy-cache plugin overrides `Cache-Control` in upstream header.

[myWsq]

Issue description

The proxy cache plugin overrides the Cache Control in the upstream header, even if the plugin is not used in the route.

apisix/apisix/cli/ngx_tpl.lua

Lines 511 to 536 in e3601e6

	{% if enabled_plugins["proxy-cache"] then %}
	### the following configuration is to cache response content from upstream server
	set $upstream_cache_zone off;
	set $upstream_cache_key '';
	set $upstream_cache_bypass '';
	set $upstream_no_cache '';
	set $upstream_hdr_expires '';
	set $upstream_hdr_cache_control '';
	proxy_cache $upstream_cache_zone;
	proxy_cache_valid any {% if proxy_cache.cache_ttl then %} {* proxy_cache.cache_ttl *} {% else %} 10s {% end %};
	proxy_cache_min_uses 1;
	proxy_cache_methods GET HEAD;
	proxy_cache_lock_timeout 5s;
	proxy_cache_use_stale off;
	proxy_cache_key $upstream_cache_key;
	proxy_no_cache $upstream_no_cache;
	proxy_cache_bypass $upstream_cache_bypass;
	proxy_hide_header Cache-Control;
	proxy_hide_header Expires;
	add_header Cache-Control $upstream_hdr_cache_control;
	add_header Expires $upstream_hdr_expires;
	add_header Apisix-Cache-Status $upstream_cache_status always;
	{% end %}

Disabling the plugin in config.yml solves this problem, but some routes need to enable the plugin. How should I preserve the Cache Control in the upstream header with the proxy cache plugin globally enabled?

Environment

• apisix version: 2.2
• OS: Linux 7cc563f53b08 4.19.121-linuxkit change: added doc of how to load plugin. #1 SMP Tue Dec 1 17:50:32 UTC 2020 x86_64 Linux
• OpenResty / Nginx version: nginx version: openresty/1.19.3.1

[spacewander]

             proxy_hide_header                   Cache-Control; 
             proxy_hide_header                   Expires; 
             add_header      Cache-Control       $upstream_hdr_cache_control; 
             add_header      Expires             $upstream_hdr_expires; 
             add_header      Apisix-Cache-Status $upstream_cache_status always; 

We can remove them from the Nginx conf, and implement them in

apisix/apisix/plugins/proxy-cache.lua

Line 251 in aff1a90

function _M.header_filter(conf, ctx)

PR is welcome!