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_ClrHeader(struct http *to)
{

	CHECK_OBJ_NOTNULL(to, HTTP_MAGIC);
	to->nhd = HTTP_HDR_FIRST;
	to->status = 0;
	to->protover = 0;
	to->conds = 0;
	memset(to->hd, 0, sizeof *to->hd * to->shd);
}

http_ClrHeader(struct http *to)
{

	CHECK_OBJ_NOTNULL(to, HTTP_MAGIC);
	to->nhd = HTTP_HDR_FIRST;
	to->status = 0;
	to->protover = 0;
	to->conds = 0;
	memset(to->hd, 0, sizeof *to->hd * to->shd);
}
