MINOR: hlua/h1: Use http_parse_cont_len_header() to parse content-length value

MINOR: hlua/h1: Use http_parse_cont_len_header() to parse content-length value

Till now, h1_parse_cont_len_header() was used during the H1 message parsing and
by the lua HTTP applets to parse the content-length header value. But a more
generic function was added some years ago doing exactly the same operations. So
let's use it instead.