* module/web/http.scm (default-val-validator): Valid with no value.
(key-value-list?): Keys are always symbols, do not accept strings.
(validate-param-list): Apply `valid?' to list elements.
(validate-credentials): Validate param for Basic scheme, which
is parsed as a string.
(declare-symbol-list-header!): `list-of?' args were in wrong order.
("Cache-Control"): Replace `default-val-validator' with more
specific procedure.
("Accept"): Validate on first param which has no value.
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