Check for duplicate Content-Length headers in requests

If a duplicate CL header is in the request, we fail the request with a
400 (Bad Request)

Fix a test case that was sending duplicate CL by misstake and would
not fail because of that.
http_IsHdr(const txt *hh, const char *hdr)
{
	unsigned l;

	Tcheck(*hh);
	AN(hdr);
	l = hdr[0];
	assert(l == strlen(hdr + 1));
	assert(hdr[l] == ':');
	hdr++;
	return (!strncasecmp(hdr, hh->b, l));
}

http_IsHdr(const txt *hh, const char *hdr)
{
	unsigned l;

	Tcheck(*hh);
	AN(hdr);
	l = hdr[0];
	assert(l == strlen(hdr + 1));
	assert(hdr[l] == ':');
	hdr++;
	return (!strncasecmp(hdr, hh->b, l));
}
