Fix graceful shutdown for Http(1|2)Builder #301

Job	Run time
Check Style	16s
features	47s
Test with Miri	42s
Check MSRV (1.63)	2m 18s
Test stable on ubuntu-latest	34s
Test stable on windows-latest	1m 18s
Test stable on macOS-latest	1m 19s
Test beta on ubuntu-latest	37s
Test beta on windows-latest	1m 21s
Test beta on macOS-latest	59s
Test nightly on ubuntu-latest	38s
Test nightly on windows-latest	1m 32s
Test nightly on macOS-latest	1m 5s
Build docs	20s
CI is green	1s
	13m 47s

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