fix: after updating the header, get the old value from the ctx.var (#… #192

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	26m 34s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	27m 39s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	25m 11s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	23m 39s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	27m 19s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	27m 43s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	25m 16s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	25m 52s
	3h 29m 13s

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