http://tools.ietf.org/html/rfc7231#section-8.1.3,

mg_http_parse_headers(s, end, hm->headers,
                      sizeof(hm->headers) / sizeof(hm->headers[0]));
if ((cl = mg_http_get_header(hm, "Content-Length")) != NULL) {
    hm->body.len = (size_t) mg_to64(*cl);
    hm->message.len = req_len + hm->body.len;
}

// mg_http_parse() is used to parse both HTTP requests and HTTP
// responses. If HTTP response does not have Content-Length set, then
// body is read until socket is closed, i.e. body.len is infinite (~0).
//
// For HTTP requests though, according to
// http://tools.ietf.org/html/rfc7231#section-8.1.3,
// only POST and PUT methods have defined body semantics.
// Therefore, if Content-Length is not specified and methods are
// not one of PUT or POST, set body length to 0.
//
// So, if it is HTTP request, and Content-Length is not set,
// and method is not (PUT or POST) then reset body length to zero.