fix(@angular/ssr): improve header validation logic #9913

Job	Run time
lint	2m 3s
build	3m 6s
build-e2e-windows	3m 6s
test	1s
e2e-snapshots	1s
Browser Compatibility Tests	1s
publish-snapshots	1s
e2e-package-managers	1s
e2e	1s
e2e-windows	1s
	8m 22s

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