Review and changes from Jocelyn Fiat

this was not a good name neither for a function
it could have bee

header_value_by_key (h, k)
or even
header_value (h, k)

since it is obvious that it is associated with a key `k'
not obvious we get it from `h', but the comment helps here
at worst we could have

header_value_from (key, source): ...