fix(brotli-plugin): Skip brotli compression for upstream compressed response #15833

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	29m 48s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	29m 41s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	20m 25s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	22m 6s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	28m 52s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	28m 5s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	22m 7s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	22m 20s
	3h 23m 24s

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