fix: content length assertion for HEAD #2907

Job	Run time
Verify hyper.h is up to date	16s
semver	41s
Check Style	4s
Test with Miri	40s
Test stable on ubuntu-latest	23s
features	22s
Test C API (FFI)	16s
Test stable on windows-latest	14s
Test cargo-c support (FFI)	19s
Test stable on macOS-latest	26s
minimal-versions	38s
Test beta on ubuntu-latest	25s
Check MSRV	30s
udeps	31s
Test beta on windows-latest	19s
Test beta on macOS-latest	35s
Test nightly on ubuntu-latest	29s
Test nightly on windows-latest	20s
Test nightly on macOS-latest	30s
Build docs	0s
Check exposed types	0s
CI is green	0s
	7m 58s

Provide feedback