fix: after updating the header, get the old value from the ctx.var #13224

Job	Run time
run ci on centos7 (lua-resty-worker-events, t/plugin/[a-k]*)	32m 47s
run ci on centos7 (lua-resty-worker-events, t/plugin/[a-k]*)	32m 47s
run ci on centos7 (lua-resty-worker-events, t/plugin/[l-z]*)	32m 31s
run ci on centos7 (lua-resty-worker-events, t/plugin/[l-z]*)	35m 0s
run ci on centos7 (lua-resty-worker-events, t/admin t/cli t/config-center-yaml t/control t/core t...	24m 24s
run ci on centos7 (lua-resty-worker-events, t/admin t/cli t/config-center-yaml t/control t/core t...	24m 24s
run ci on centos7 (lua-resty-worker-events, t/node t/pubsub t/router t/script t/secret t/stream-n...	23m 22s
run ci on centos7 (lua-resty-worker-events, t/node t/pubsub t/router t/script t/secret t/stream-n...	23m 22s
run ci on centos7 (lua-resty-events, t/plugin/[a-k]*)	31m 53s
run ci on centos7 (lua-resty-events, t/plugin/[a-k]*)	31m 53s
run ci on centos7 (lua-resty-events, t/plugin/[l-z]*)	33m 15s
run ci on centos7 (lua-resty-events, t/plugin/[l-z]*)	33m 54s
run ci on centos7 (lua-resty-events, t/admin t/cli t/config-center-yaml t/control t/core t/debug ...	25m 1s
run ci on centos7 (lua-resty-events, t/admin t/cli t/config-center-yaml t/control t/core t/debug ...	25m 1s
run ci on centos7 (lua-resty-events, t/node t/pubsub t/router t/script t/secret t/stream-node t/u...	23m 7s
run ci on centos7 (lua-resty-events, t/node t/pubsub t/router t/script t/secret t/stream-node t/u...	23m 7s
	7h 35m 48s

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