Add the conditional response with cache entity tag

[u5surf]

• We can reply 304 response with cache to possible to reduce body response if cache entity tag is equivalent to client side one.
• We can do that to use the request with If-None-Match.
• https://www.rfc-editor.org/rfc/rfc7232#section-2.3

Nginx has the same mechanism can reply the 304 response using cache.

https://github.com/nginx/nginx/blob/e3207a17f084c9eb7905ca0f2cfdb1df088fd165/src/http/ngx_http_upstream.c#L6023-L-6043

https://github.com/nginx/nginx/blob/e3207a17f084c9eb7905ca0f2cfdb1df088fd165/src/http/modules/ngx_http_not_modified_filter_module.c#L86-L105

[kevinmcconnell]

Thanks @u5surf, this looks great 👍