fix: add post request headers only if auth request method is post #69

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/http3/admin)	12m 40s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	29m 32s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	30m 59s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	22m 24s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	22m 18s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/http3/admin)	11m 29s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	31m 10s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	31m 18s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	21m 10s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	21m 25s
	3h 54m 25s

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