fix(traffic-split): `upstream_obj.upstream` should not be a string (#… #1062

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/http3/admin)	15m 23s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	36m 50s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	34m 52s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	25m 32s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	25m 53s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/http3/admin)	16m 11s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	36m 5s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	34m 54s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	26m 49s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	28m 26s
	4h 40m 55s

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