fix: accept generic header in meta builder (#30) #108

Job	Run time
doctest	31s
docs	46s
cfg-check	46s
clippy	21s
feature-checks	36s
no-std	7s
fmt	11s
test stable --no-default-features	40s
test stable	44s
test stable --all-features	35s
test nightly --no-default-features	1m 11s
test nightly	1m 12s
test nightly --all-features	1m 11s
test 1.79 --no-default-features	33s
test 1.79	45s
	10m 9s

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