Issue description

If proxy-cache is configured in plugin list, it will override Cache-Control and Expires headers from the upstream.
Problem is that even if proxy-cache is not configured in any route, those headers will still be overridden.
So we have a side effect for proxy-cache.

Environment

• apisix version (cmd: apisix version): 2.0
• OS (cmd: uname -a): linux
• OpenResty / Nginx version (cmd: nginx -V or openresty -V): openresty/1.19.3.1
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API):
• apisix-dashboard version, if have:

Minimal test code / Steps to reproduce the issue

1. Use the default configuration from apisix
2. Write Cache-Control and Expires headers in upstream
3. Check the response

What's the actual result? (including assertion message & call stack if applicable)

Cache-Control and Expires headers disappeared

What's the expected result?

If proxy-cache is not configured for a route, apisix should not rewrite Cache-Control and Expires headers.