fix: content length assertion for HEAD #2909

Job	Run time
Check Style	7s
semver	12s
Verify hyper.h is up to date	19s
minimal-versions	48s
Test C API (FFI)	27s
features	32s
Test cargo-c support (FFI)	17s
Test with Miri	2m 22s
Check MSRV	28s
udeps	19s
Test stable on ubuntu-latest	1m 17s
Test stable on windows-latest	2m 43s
Test stable on macOS-latest	1m 27s
Test beta on ubuntu-latest	1m 27s
Test beta on windows-latest	2m 22s
Test beta on macOS-latest	1m 43s
Test nightly on ubuntu-latest	1m 28s
Test nightly on windows-latest	2m 32s
Test nightly on macOS-latest	1m 41s
Check exposed types	20s
Build docs	23s
CI is green	1s
	23m 15s

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